In this chapter, we focus on functions of more than one variable. Primarily, the focus is on functions of two and three variables, called bivariate and trivariate functions, respectively. We look at derivatives and integration over these functions, which are useful for volume calculations. Also presented in this chapter are quadratic forms, which are simple but special bivariate functions that appear frequently in science and engineering applications. Continuing the discussion from Section 10.2, we introduce a bivariate Newton-Raphson method for finding intersections. We continue the discussion of PDEs as well, and look at some special bivariate functions defined by them.