This document provides instruction on trigonometric identities. It defines an identity as a mathematical statement that is always true for the given variables. Examples are provided of using fundamental trigonometric identities to simplify expressions and prove other identities. Applications of identities to rewriting trigonometric expressions and solving a physics problem about the angle at which an object will start to slide down an incline are also demonstrated. Students are guided to use identities involving sine, cosine, tangent, cotangent, secant and cosecant functions.