Place two linked portals to teleport yourself and objects. Use momentum - 'flinging' through portals builds speed to reach high areas.
The campaign has 10 chapters with increasingly complex puzzles involving lasers, light bridges, gels, and tractor beams.
Play as Atlas and P-Body with a friend. Each player has their own portal gun, requiring teamwork to solve puzzles.
Chapter 3 teaches the emancipation grill: anything that enters it is destroyed, so never fire a portal into an emitter or into a surface that a grill covers. Chapter 4 introduces the aerial faith plate - step on the painted target and let it launch you, then chain a second plate mid-air to clear the chasm before the turrets finish warming up. Chapter 6's laser relay needs the redirection cube: aim the beam into the cube, then into the receptor, and the door unlocks.
Chapter 7 brings Repulsion Gel for bounce and Propulsion Gel for speed. Repulsion Gel only bounces when you hit the surface already moving downward; Propulsion Gel stacks speed, so one running jump carries you across an entire test chamber. Conversion Gel is the only surface portals stick to in chapter 8 - paint the tube with it, then place both portals on the painted patch.
Flinging is the most useful trick in the game: put one portal high, one low, fall through the low one and exit the high one with horizontal momentum. In the final chapter, shoot the moon - literally. When the core is exposed, the only remaining surface is the lunar one, and firing a portal at it ends the fight.
Co-op assumes two players: Atlas and P-Body each carry four portals. The mass and velocity course needs one player to hold a cube on a button while the other flings through a shared portal. In the laser course, time both beams into the same receptor - the window is roughly one second.