Here is the full list of all 37 Psychonauts achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Redeem all 16 Scavenger Hunt Items.
Discover the secret history of Whispering Rock.
Find Edgar's Secret Garden.
Uncover Milla's Secret.
Complete Meat Circus.
Complete The Asylum.
Re-brain the Children.
Achieve Rank 80.
See yourself through the eyes of many others.
Complete Black Velvetopia.
Earn all Merit Badges.
Complete Waterloo World.
Use the bacon. A lot.
Complete Gloria’s Theater.
Complete The Milkman Conspiracy.
Achieve Rank 60.
Complete Lungfishopolis.
Hear Vernon's Ghost Story.
Spy on Bobby's love life.
Read many bulletin board messages.
Complete Milla’s Dance Party.
Achieve Rank 40.
Complete Sasha’s Shooting Gallery.
Achieve Rank 20.
Witness Maloof's transformation.
Cook and consume two different kinds of roast in one sitting.
Complete Basic Braining.
Achieve Rank 101.
Sort all Emotional Baggage.
Crack all Vaults.
Gather all Figments.
Find all Golden Helmets.
Introduce all Camp Kids to Mr. Pokeylope.
Revisit all brains after completion.
Complete the Punchy Target mini-game.
Give the Coach's speech on the stump.
Buy an item from the Camp Store on Christmas.