What Engine Does Apex Legends Run On?

The Apex Legends engine is a heavily modified version of Source, Valve's engine that also underpins Half-Life 2 and Counter-Strike. Respawn inherited it from Titanfall, which ran on the same modified branch before Apex Legends did.

Why Source, and why it still works

Respawn's engineers had already spent years bending Source toward fast-paced shooter movement for Titanfall's wall-running and mechs. Reusing that base for Apex Legends meant the studio was building a battle royale on tooling it already knew well, rather than starting from nothing.

How much has changed underneath it

"Modified" is doing real work in that description. Respawn has rebuilt large parts of the engine over the game's life to support 60-player matches, seasonal map changes and destructible terrain — none of which the original Source engine was built to handle at that scale.

The exception: mobile

Apex Legends Mobile, the standalone spin-off Respawn built with Tencent's Lightspeed and Quantum Studios, ran on Unreal Engine 4 instead — a separate codebase built for phone hardware. That version was shut down on 1 May 2023, so Unreal Engine 4 is no longer part of any Apex Legends product currently running.

The takeaway

Every version of Apex Legends you can play today, on PC or console, runs on the same modified Source lineage that started with Titanfall.