Croteam created their own engine for use in both First Encounter and Second Encounter. Named the "Serious Engine", it is designed to cope with extremely large view distances and massive numbers of models, unlike most FPS engines which are developed for a limited draw distance and only a few animating models (i.e. enemies) on screen at a time: it is very efficient, capable of maintaining dozens of moving enemies (often stampedes) even on a modest system challenging the well known Quake, Unreal or Half-Life engines. The "Serious Engine" can render through both Direct3D or OpenGL and, while it does not support pixel or vertex shaders, it is optimised for Direct3D 7's hardware transformation, clipping and lighting. The "Serious Engine" has been also available as a licensed engine from Croteam.
The next, more powerful iteration of the "Serious Engine" has been developed for use in Serious Sam II and has been called "Serious Engine II".This one has support for many of the features of modern GPUs such as pixel shaders,vertex shaders HDR,bloom and Parallax map
ping.
The next, more powerful iteration of the "Serious Engine" has been developed for use in Serious Sam II and has been called "Serious Engine II".This one has support for many of the features of modern GPUs such as pixel shaders,vertex shaders HDR,bloom and Parallax map
ping. 
