What distinguises a ``good program'' from a ``bad program''? Obviously, a good program has to work correctly and reliably. But this is far from enough. In practice, very few programs are written once, used for a while, and discarded: much more often, a program is used until the need for it changes, the program is modified to handle the new requirements, the modified program is used for a while, and the cycle repeats. Thus a ``good program'' must be not only correct the first time around, but structured in such a way that it can easily be modified to accomodate likely changes in requirements. The implications of this fact will follow you throughout your years as a computer science student and your career as a computer professional; we'll only scratch the surface this semester.