This chapter discusses how to write the sorts of Prolog programs that will be used in the rest of the book. Section 1 examines what it means for a program to be fully correct. Section 4.2 introduces a new program in a blocks world and studies how it was written. This leads to a discussion of recursion in Section 4.3, and to its companion, mathematical induction, in Section 4.4. Section 4.5 considers the issue of programs that run forever and how to avoid writing them. Section 4.6 looks at a more complex predicate as it appears in the blocks-world program. Finally, Section 4.7 introduces the issue of program efficiency.