Most oPfpthe chapters of the present book can be read independently of each other. They fall into groups: Chapters 1-6 deal with polynomial and spline approximation - in some sense they continue the themes of [CA]. Chap- ters 7-10 contain a fairly complete theory of rational approximation. Chapters 12-14 treat widths and entropy of classes of functions. But even within the groups, chapters are more or less independent, except that it is advisable to read Chapter 3 before Chapter 4, while Chapter 7 is indispensable for Chap- ters 8 and 10, and Chapter 13 for Chapter 14. Most of the information about Banach function spaces needed in the two volumes of CA can be found in [CA, Chapter 2], in 7 of Chapter 1 of the present volume, and in the book of Bennett and Sharpley [B-1988]. We also provide a quick new look at some of the important approximation theorems: for polynomials in 7 of Chapter 1, for splines in 1 of Chapter 6.