Large area (≈1 cm2) silicon avalanche photodiodes (SiAPDs) have been fabricated and their performance as optical detectors for use with scintillating crystals has been measured. Light sensitivity is measured for hexagonal SiAPDs of 1.57 cm2 total package area, and the energy and timing resolution is measured for these devices coupled to CsI(Tl) scintillators operating as gamma spectrometers. © 1990.