In ARL's current chirped amplitude modulation (AM) ladar prototypes using unity gain solid state detectors, amplifier noise limits the receiver sensitivity. This noise is well above the signal shot noise limit. We are developing a method using Geiger-mode avalanche photodiode (Gm-APD) photon counting detectors in the chirped AM ladar receiver to yield sensitivities approaching the signal shot noise limit. This method is based on the fact that the chirped AM waveform that modulates the transmitted laser power also modulates the average photon arrival rate at the receiver with a delay corresponding to the round-trip time between the transceiver and the target. We present the concept, and computer and electronic simulation results for ARL's chirped AM ladar operating with a photon counting receiver. We also present the design and initial results of the proof-of-principle laboratory optical experiment that we recently performed. The simulation and experimental results predict significant improvements in the ladar's receiver sensitivity.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.