Program slicing is a method for decomposing programs into slices. A program slice contains all statements that could have influenced the value of a variable of interest at some program point. The technique of program slicing has up to now been applied within a number of areas, such as software maintenance, program integration, testing, parallel processing distribution and, especially, program debugging. A slice of a program is computed either by use of static information (a static slice) or dynamic information (a dynamic slice). A static slice of a program, although imprecise, contains much broader information, and thus it tends to be much thicker than the corresponding dynamic slice. The reason is that, for a specified variable in a program, the static slice is the collection of all possible computations of values of that variable. On the other hand, a dynamic slice isolates the unique computation of the value of the specified variable for a certain program execution. The current literature on slicing is becoming unwieldy, with many different notions and methods. This article attempts to bring order to disorder by identifying and classifying different notions of program slicing and their applications. A new notion of execution slice and a method for interprocedural dynamic slicing are introduced as well.