Some functions have them and some don’t. Some have them at some points and not at others. This chapter is on the derivative. Functions which have derivatives are somehow better than those which don’t. To begin with it is necessary to discuss the concept of a limit of a function. This is a harder concept than continuity and it is also harder than the concept of the limit of a sequence or series although that is similar. One cannot make any rational sense of the concept of derivative without an understanding of limits of a function.