The Brookhaven Accelerator Test Facility (ATF) has provided users with a facility to study complex properties of modern particle accelerators, new methods of accelerating electrons and ions, and fundamental plasma physics for over two decades. The facility was originally designed as a test bench for accelerator research critical to the advent of the free electron laser, with first demonstrations of high brightness electron beams from photocathode RF guns, self-amplified seeded emission in visible wavelengths, and seeded high gain harmonic generation. Today, the ATF specializes in providing multi-beam capabilities with a unique terawatt-class, picosecond, 9.2 µm CO2 laser synchronized with the 80 MeV electron beam from the linac and a femtosecond titanium-sapphire laser. This has enabled research in new regimes of plasma physics, laser-based particle acceleration, and radiation sources.