Abstract. Interactive programming is a method for implementing programming languages that supports an interactive, exploratory style of program development and debugging. The basic idea is to reify the steps of a computation into a persistent data structure which can be explored interactively, and which reacts to changes to inputs like a spreadsheet. Reifying the computation associates the computed value with provenance information, which is essential to effective program comprehension and debugging. Making the data structure persistent means that it can evolve incrementally, preserving existing structure where possible, allowing the programmer to apply fixes to a program in the middle of a complex debugging activity without having to restart the program and lose browsing context. Interactive programming lies at the intersection of incremental computation, software visualisation and reactive programming.