Remote sensing of the atmosphere using lasers is referred to as lidar, an acronym for "light detection and ranging" and is analogous to radar. The transmission of short pulses of laser light is followed by reception of backscattered signals from atmospheric constituents such as aerosols, dust, clouds or molecules. Information about the composition and physical state of the atmosphere can be deduced from the lidar data. In addition, the time lag of the received signal determines the range of the scattering species. This research is to derive and calculate the received power from a lidar system, both coherent and direct detection. The effects of pulse shape, turbulence, transmitter, receiver and local oscillator will be determined; the effects of the motion of the atmosphere will be investigated; and physics of the important mechanisms will be identified. There is a regime where turbulence is the dominating physical mechanism. The feasibility of estimating turbulence levels from lidar measurements in this regime will also be investigated. The theoretical results will be obtained from a recent series solution for statistics of waves propagating in random media. This provides a solution for very general conditions with a minimum of approximations. Based on these results, comparison between coherent and direct detection will be performed and an apparent error in the lidar equation for coherent detection will be investigated. The feasibility of deducing turbulence profiles from lidar measurements will also be investigated.