Pass the toilet paper to the Scientist in need.
Perform surgical caffeine extraction from 20 vending machines.
Kill 10 airborne Headcrabs using the Shotgun.
Kill 20 Enemies with the Hivehand.
Ruin the microwave casserole.
Kill an Assassin with the .357 Magnum.
Kill 10 enemies with a reprogrammed Sentry.
Conduct all of the questionable research experiments.
Kill 10 Enemies with Snarks.
Headshot 20 Enemies with the .357 Magnum.
Drown in Lambda Core's coolant.
Kill a Headcrab using fire.
Kill a Marine with their own Grenade.
Kill 5 Enemies with one Grenade.
Kill an Ichthyosaur with either the Tau or Gluon.
Use the alternate path in Office Complex.
Use an entire clip of Gluon ammo in one continuous shot.
Get overtaken by a Headcrab's ample anterior.
Interfere with the alarm and laptop in Anomalous Materials.
Overcharge it anyway.
Send the Hidden Hat to Xen.
Kill a Barnacle with a Satchel.
Refuse to insert the specimen into the Anti-Mass Spectrometer.
Help the Barnacle and Headcrab find true love.
Get killed by your own Snarks.
Kill all Protozoans in the Borderworld.
Don't kill any Vortigaunts in the Borderworld.
Carry the Hidden Hat to Endgame.
Kill 10 Enemies with Tripmines.
Escape the Hangar in Surface Tension without alerting the Soldiers.
Wait too long to jump into the Lambda Teleporter.
Assemble all the possible Scientists and Guards at the end of Office Complex.
Die in the AMS Beam before starting the experiment.
Skip the opening Xen island with a secret jump.
Use the welcome mat.
Deliver the finishing blow to Gonarch with your Crowbar.
Get whacked into oblivion in the ledge fight.
Find each member of the missing research teams.
Use Cyanogen to kill Gonarch.
Don't use the Cyanogen and let the Gonarch live.
Find the snack supply canister.
Complete Interloper without taking any laser damage.
Escape the Gargantua’s Lair without taking any damage from Gargantuas.
Kill a Controller without killing any of its mind-controlled Vortigaunt slaves.
Take out all three Healing Crystals in Nihilanth’s Chamber at the same time.
Defeat Nihilanth in under 4 minutes.
Accept G-Man’s offer of employment.
Decline G-Man’s offer of employment.
View the *other* secret ending.
Cook a Grenade for too long.