The material in this Field Guide includes a review of classical Kolmogorov turbulence theory, Gaussian-beam waves in free space, and atmospheric effects on a propagating optical wave. These atmospheric effects have great importance in a variety of applications like imaging, free space optical communications, laser radar, and remote sensing. This Guide presents tractable mathematical models from which the practitioner can readily determine beam spreading, beam wander, spatial coherence radius (Fried's parameter), angle of arrival fluctuations, scintillation, aperture averaging effects, fade probabilities, bit error-rates, and enhanced backscatter effects, among others.