Geiger mode avalanche photodiodes (GAPDs) are capable of detecting single photon events. However, once triggered, GAPDs must be reset or rearmed to enable the detection of another event. Thus, these devices are non-linear and their performance depends on the reset-time a.k.a. dead-time. In this paper, the performance of GAPD based ladar receivers is investigated and a theory for the signal photon detection efficiency (SPDE) is developed as a function of the dead-time; signal, noise and clutter flux; and the GAPD's photon detection efficiency or PDE. This SPDE theory is valid for arbitrary (short to long) dead-times. With a zero dead-time, GAPDs behave linearly and the SPDE theory converges to the PDE. For long dead-times, compared to the acquisition gate time, the theory converges to previously published works of Fouche and Williams. This SPDE theory is then applied to develop a theory for the detector signal-to-noise ratio (SNR). The performance improvement when multiple micro-pixels are grouped to form a macro pixel is also discussed.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.