There is a conspicuous gap in plans for X-ray timing after the X-ray Timing Explorer (XTE). Timing science has played a critical role in the development of X-ray astronomy. The need now is to move into a new domain of shorter timescales and weaker modulation, one that can be reached only with very large aperture instruments. XLA is an X-ray facility with an aperture substantially greater than 1 m2, nominally 100 m2. Most of this area is devoted to a large array of collimated proportional counters. There is also a ~ 1 m2 coded aperture. It extends observational parameter space by several orders of magnitude in timing resolution, sensitivity to variability, and angular resolution. This will lead to a qualitatively new kind of X-ray astrophysics that can be applied to the study of a broad range of astrophysical objects. XLA is thus both an advanced timing mission and a general purpose facility whose principal uses are in areas that are not well covered in other aspects of the planned High Energy Astrophysics program.