Which video game engine powers Portal?

Portal is powered by Valve’s Source game engine.

Source is a 3D engine developed by Valve and first released commercially in 2004. It succeeded GoldSrc, the heavily modified Quake-based technology behind earlier Valve games such as Half-Life and Counter-Strike. Portal uses Source’s rendering, collision detection, physics, audio, scripting, and level-loading systems.

The engine matters to Portal because its physics make the portal mechanic work convincingly. Objects retain momentum as they pass between linked openings, while the orientation of the destination portal changes their trajectory. That lets players build speed by falling through one portal and launch themselves from another, a technique commonly called “flinging.”

A common mix-up is confusing Source with GoldSrc or Source 2. GoldSrc predates Source, while Source 2 is Valve’s later engine family. Portal is specifically listed as a Source-engine game, alongside titles such as Half-Life 2, Team Fortress 2, and Left 4 Dead. Portal 2 also uses Source, although it contains substantial additions and refinements built for that sequel.