The theory of spline functions and their applications is a relatively recent development. As late as 1960~ there were no more than a handful of papers mentioning spline functions by name. Today~ less than 20 years later, there are well over 1000 research papers on the subject, and it remains an active research area. The rapid development of spline functions is due primarily to their great usefulness in applications. Classes of spline functions possess many nice structural properties as well as excellent approximation powers. Since they are easy to store~ evaluate~ and manipulate on a digital computer, a myriad of applications in the numerical solution of a variety of problems in applied mathematics have been found. These include~ for example~ data fitting, function approximation numerical quadrature, and the numerical solution of operator equations such as those associated with ordinary and partial differential equations, integral equations, optimal control problems, and so on. Programs based on spline functions have found their way into virtually every computing library.