This document defines circles geometrically as the result of a cone intersecting with a plane, and algebraically as the set of points equidistant from a fixed center point. It provides the standard equation for a circle given the center (h,k) and radius r: (x-h)2 + (y-k)2 = r2. Examples are given of writing the equation of a circle and finding its center and radius given parts of the equation. The process for finding the equation of a circle given its diameter is also described.