This paper begins with a discussion of the factors affecting emission, transmission, reflection, and absorption of light, including most importantly, the role of the atmosphere. It continues with a discussion of various applications, including detection of point targets and imaging of extended targets. The use of EO/IR for tracking targets is also covered, as are some recent technological advancements. Finally, sections are dedicated to the topics of sampling, aliasing, and reconstruction, as well as issues in color cameras.