The effect of signal and background shot noise, as well as device noise, on the performance of a direct-detection spatial tracking system is investigated for arbitrary detector arrays, assuming linear loop operation. The performance of quadrant detectors as a function of background radiation and detector radius is then analyzed and compared to some performance bounds. Tracking systems based on avalanche photodetectors (APD) and PIN detectors also are compared. The effects of non-focal-plane processing, focus error, and pupil walk on tracking performance are investigated. Experimental results of a 2.4 kHz two-axis tracking loop operating at low signal power using an APD quadrant detector are presented.