D programs consist of a set of clauses that describe probes to enable and predicates and actions to bind to these probes. D programs can also contain declarations of variables, as described in Variables, and definitions of new types, described in Type and Constant Definitions. This chapter formally describes the overall structure of a D program and features for constructing probe descriptions that match more than one probe. We'll also discuss the use of the C preprocessor, cpp, with D programs.