Alien Swarm: Complete Guide

Everything you need to get through Valve's free top-down co-op shooter: classes, loadouts, the Jacob's Rest campaign, difficulty settings, and when to jump to the community sequel, Reactive Drop.

Researched September 2026 — current as of the final Valve update (August 18, 2020). Live games change; check the linked sources if something differs.

Where Alien Swarm Stands in 2026

Alien Swarm launched free on Steam on July 19, 2010, together with its full source code and SDK. Valve added Onslaught mode, Brutal difficulty and Hardcore Friendly Fire on August 23, 2010, and the last official patch (August 18, 2020) was a maintenance update: security fixes for client/server file transfers, a fix for the "Out of thread ids" launch error on PCs with 32 or more logical processors, and updated localization.

The game is still free and still launches, but the public matchmaking population is small. Most people now play Alien Swarm: Reactive Drop (Steam app 563560), a free fan continuation released April 20, 2017 that adds new campaigns, bots for solo play, up to 8-player co-op, PvP modes (Deathmatch, Gun Game, Team Deathmatch), Workshop support and 100+ achievements. Everything in this guide about classes, aliens and teamwork carries over to Reactive Drop.

Quick Start

You need a squad. The original game has no bots. Play with friends (4 players) or join public lobbies; for solo play with bots, install Reactive Drop.
Bring one of each class. Officer, Special Weapons, Medic and Tech each fill a real role. The Tech is mandatory in practice because doors and terminals need hacking.
Friendly fire is always on. Do not shoot through teammates. It is one of the most common ways runs end.
Learn the fast reload. When the reload bar appears, press reload again as the marker crosses the highlighted zone. There are achievements for it (Quick Load, Gunslinger) and it matters a lot on Hard and above.
Protect the Tech during hacks. The hacker is locked into a minigame and cannot shoot. Form a ring around them.
Kill Swarm eggs and Parasites fast. A Parasite that latches on infests a marine; the Medic must cure it or the marine dies.
Use the welder on doors. Sealing a door behind you buys time when the swarm is chasing you.
Level up to unlock gear. Experience after every mission unlocks new weapons and equipment; your first hours are partly about building an arsenal.
Start on Normal. Easy teaches the maps; Normal teaches the mechanics; Hard is where coordination starts to matter.

Classes and Marines

There are four classes, each with two selectable marines. Pick based on the passive abilities shown on the briefing screen.

Class	Marines	Role	What only they can do
Officer	Sarge, Jaeger	Frontline damage and squad buffs	Leadership-style bonuses to nearby squadmates; strong all-round combat
Special Weapons	Wildcat, Wolfe	Sustained heavy fire	Carries the Autogun (and later the Minigun); smart-aim heavy guns chew through waves
Medic	Faith, Bastille	Healing and cures	Heals marines and cures Infestation; the only real sustain in the squad
Tech	Crash, Vegas	Hacking and utility	Hacks computers and wired door panels, runs the scanner, fast-deploys utilities

How to play each class

Officer: Walk point with the Assault Rifle or Shotgun. Your job is clearing the way and holding chokes. You are often the one to call pushes.
Special Weapons: Stay slightly behind the Officer and cover the widest arc. The Autogun's assisted aim makes it the best general-purpose swarm clearer in the game, but ammo goes fast; ask the Tech or Officer for an ammo stash.
Medic: Stay in the middle of the formation, not the back. Watch health bars and the Infestation icon. A Heal Beacon placed at a defense point heals everyone standing in it (the Group Heal achievement is for healing all four with one beacon).
Tech: Pick a loadout that still lets you fight (PDWs or a Prototype Rifle), because you are the target while hacking. Carry a sentry gun or ammo for the team.

Weapons and Equipment

Over 40 weapons and items unlock as you level. The names below are all confirmed by the game's achievement list.

Weapon/Item	Notes
Assault Rifle	Reliable default; underbarrel grenades. Good on any class that can carry it.
Prototype Rifle	Strong rifle unlocked by levelling.
Autogun	Special Weapons signature gun; assisted aim, huge magazine.
Minigun	Heavier Special Weapons option; spin-up, massive DPS.
Shotgun	Close range; great for Officers holding doors.
Vindicator	Heavy-hitting automatic shotgun; great for holding doorways.
Flamethrower	Area denial; very easy to torch teammates with (especially under Hardcore FF).
Tesla Cannon	Chains electricity between aliens; good crowd control.
Rail Rifle	High-damage precision shots.
Marksman Rifle	Long-range, single-target.
Grenade Launcher	Burst damage into packs; watch splash near teammates.
Chainsaw	Melee-range shredder.
Twin Pistols and PDWs	Light weapons; common Tech/Medic sidearms.
Sentry Guns	Deployable turrets; excellent for holding hack points and Onslaught corridors.
Heal Beacon	Medic area heal.
Ammo stash	Deployable ammo for the squad.
Hornet Barrage	Launches homing rockets.
Laser Tripmines, Incendiary Mines	Area denial at doors and chokepoints.
Stun Grenade, Freeze Grenade, Grenades	Crowd control and burst.
Damage Amp	Temporary damage boost.
Welder	Seal or cut through doors.

Loadout rules of thumb

Each marine carries two weapons and one extra item. Take one "main job" weapon and one utility.
Double up on area control on higher difficulties. Tesla, mines and sentries make hack defenses far easier.
Do not bring the Flamethrower to a pub. In unfamiliar groups it causes more teamkills than kills.
Medics should not skip their heal item for a second gun. The squad has nowhere else to get health.

Know Your Enemies

Alien	What to do
Drones	The basic swarm. Focus fire and keep formation; they come in waves.
Parasites	Small, fast; they latch on and infest. Kill on sight (you can even punch them, per the Parasite Puncher achievement).
Swarm Eggs	Destroy before they hatch. Clearing all eggs without any hatching earns Scrambled Eggs.
Grubs	Tiny critters; harmless alone but clutter the screen.
Shieldbug	Heavily armoured from the front. Split up and flank; hit it from behind or the sides.
Boomer	Inflates and bursts into more enemies. Kill it quickly before it inflates.
Swarm Ranger	Fires projectiles. Dodge-roll under the shots (Under the Gun achievement).

The Jacob's Rest Campaign

The single official campaign is Jacob's Rest, set in December 2052 on an overrun colony world. You push from the landing zone through surface bridges, a reactor, a residential complex, sewers and a station, ending with the squad escorting a thermonuclear bomb to finish the colony. The campaign has seven missions, each with its own speed-run achievement; they are listed below in campaign order (the in-game campaign map shows the route if you want to confirm).

SynTek Hospital is not part of Jacob's Rest. Reactive Drop has a "SynTek Hospital" mission, but its patch notes file it under the Tilarus-5 campaign, not Jacob's Rest. The original Alien Swarm has no Hospital mission.

Mission	Speed-run target (Normal+, no deaths)	Tips
Landing Bay	1:25	Tutorial-length opener. Learn hacking and door welding here.
Cargo Elevator	2:50	Holdout while the lift moves. Bunch up, face outward, drop sentries.
Deima Surface Bridge	2:30	Long, exposed route. Keep moving; do not chase stragglers.
Rydberg Reactor	3:10	Objective-heavy map ending in a holdout that includes Swarm Rangers. Whenever the Tech stops to hack, everyone else guards them.
SynTek Residential	2:30	Tight corridors. Shotguns and Autogun shine; watch friendly fire in doorways.
Sewer Junction B5	1:30	Short and hectic. Stay together through the tunnels and head straight for the escape area once the objective is done.
Timor Station	4:25	The finale. Long defensive stretches, a data download and a final hack that need the Tech alive; bring sentries, mines and a full medic loadout.

Speed-run times are for the achievements; on a first run, move steadily and clear side rooms for ammo.

Campaign achievements worth planning for

Achievement	Requirement	How to approach it
Easy/Normal/Hard Campaign	Finish Jacob's Rest on that difficulty or harder	Harder clears count for the easier ones, so a Hard clear unlocks all three.
Insane Campaign, Brutal Campaign	Finish the campaign on Insane / Brutal	Only with a fixed, voice-chatting squad.
Outstanding Execution	A whole campaign on Normal+ with no marine deaths	Play Normal, not Hard; restart a mission rather than push on after a death.
Zero Mortality, Perfect	One mission on Normal+ with no deaths / no damage taken	Landing Bay is the shortest attempt for Perfect.
Short Controlled Bursts	A mission with no friendly-fire incidents (four marines present)	Leave the Flamethrower and Grenade Launcher at home.
Hardcore	Any mission on Brutal with Onslaught and HCFF	Pick the shortest mission (Landing Bay) and bring sentries.
Nuke From Orbit	Kill 100,000 Swarm	A long-term grind; it happens naturally over many campaigns.

Difficulty, Onslaught and Hardcore FF

Difficulties: Easy, Normal, Hard, Insane, and Brutal (added in August 2010; Valve said no one on the team had completed a mission on it at the time).
Onslaught: An AI Director that spawns extra swarms dynamically based on factors such as squad stress. It works on any difficulty and makes missions play differently each time.
Hardcore Friendly Fire (HCFF): Friendly fire deals full damage immediately, flamethrowers ignite teammates faster, and sentry turrets will hurt you if you are in their line of fire.
Promotion ranks: After maxing out, you can promote; the August 2010 update added Platinum Star, Osmium Star and Iridium Medallion ranks for dedicated players.

The Hardcore achievement requires finishing any mission on Brutal with both Onslaught and HCFF enabled.

Common Mistakes to Avoid

Running ahead. Aliens flank from vents and side rooms. A lone marine gets swarmed and the Medic cannot reach them.
Shooting across your squad. Walk into firing lanes, not through them. Step aside rather than shooting past a teammate's shoulder.
Ignoring the Tech. If the Tech dies, doors and terminals cannot be opened. There are achievements for protecting the Tech (Technician Secured, Protect the Tech) for a reason.
Wasting ammo on grubs. Save magazines for drones and big threats; ask for ammo before you run dry, not after.
Letting eggs hatch. Every hatched egg is another parasite looking for a host.
Reloading at the wrong moment. Call reloads, or at least stagger them, so someone is always firing.

Advanced Tips

Fast reload every time. A consistent fast reload is effectively a DPS upgrade and a safety net.
Assign lanes. In holdouts, each marine covers one direction. The Special Weapons marine takes the widest approach.
Seal doors on retreat. The welder turns a doorway into a breather; seal a door once the squad is through and let the Tech hack in peace.
Hack cleanly. The game tracks whether your wire and computer hacks get "access logged"; the Circuit Breaker and Security Expert achievements each ask for ten hacks without it. Practice the hacking minigames on Easy.
Sentries first, then fight. On Onslaught, drop turrets as soon as a hold starts, not when the wave is already on you.
Flank Shieldbugs deliberately. Have one marine bait from the front while two strafe around.
Kill Boomers at range. Their burst spawns more enemies right into your formation.

Mods, Reactive Drop and Modern PCs

Reactive Drop is the easiest way to play in 2026: it is free, has bots, bigger lobbies, more campaigns and an active Workshop, and it is still patched (named updates such as "Going Up" in April 2026). Your Alien Swarm skills transfer directly. Jacob's Rest is included, alongside official campaigns such as Operation Cleansweep, Area 9800, Tears for Tarnor, Paranoia, Lana's Escape, Tilarus-5, Nam Humanum, Research 7, BioGen Corporation, Accident 32 and Reduction.
High core-count CPUs: The 2020 patch fixed the crash on machines with 32+ logical processors; if you had that issue years ago, update and try again.
Modding: Alien Swarm shipped with the full code base and a tile-based map generation tool, which is why so many community maps and the Reactive Drop project exist.

FAQ

Is Alien Swarm free? Yes. It has been free on Steam since launch in 2010, and Reactive Drop is also free.

Can I play Alien Swarm solo? Not properly in the original; it is built for human squads. Reactive Drop adds a single-player mode with bots.

Is anyone still playing? The original's matchmaking is quiet. Reactive Drop is where the community plays; its Steam reviews remain strongly positive.

What is the best class for beginners? Special Weapons. The Autogun's assisted aim is forgiving and your job (shoot the swarm) is simple. Move to Medic or Tech once you know the maps.

How do I cure an infested marine? The Medic has to treat them before the infestation runs its course. Kill Parasites before they latch on to avoid it entirely.

What does Onslaught do? It turns on an AI Director that adds extra waves based on how the squad is doing, making every run different.

How many achievements are there? The store lists 64 at launch; the current Steam achievement list shows 66, including the Brutal and Insane campaign awards.