This chapter provides a discussion about thin lenses and how they are treated and analyzed for optical imaging. The concept of a thin lens is useful because aberration calculation with formulas is simplified. Structural aberration coefficients are used to determine aberrations and to show the rationale on the choice of the lens shape and aperture stop location. An understanding of how a singlet lens works is indispensable for the design of complex lens systems. The Wollaston periscopic lens, or landscape lens, is discussed regarding the technique of artificially flattening the field of view. A simple optical relay system is discussed, and then complexity is added to correct the primary monochromatic aberrations.