This unit begins with the definition of a function, a special relationship such that for all x-values there exists one and only one y-value. Using this definition, functions are evaluated for given values. These x- and y-values, or inputs and outputs, are grouped into domain and range respectively. Domain and range are discussed both graphically and algebraically. Graphing and writing piecewise functions combines equations with domain while deepening the understanding of domain and range. The idea of combining functions is also carried out in operations on functions, including addition, subtraction, multiplication, and division. The composite operation is introduced in Algebra II. This study of function operations will lead to an investigation of derivatives of sums, differences, products, and quotients of functions in Calculus.