Labelled graphs are used to model control and data flow among events occurring in the execution of a (possibly concurrent) program. Data flow is a unifying concept that covers both access to memory and communication along channels; it covers many variations including weakly consistent memory, re-ordered execution, and communication channels that are multiplexed, buffered, or otherwise unreliable. Nevertheless, the laws of Hoare and Jones correctness reasoning remain valid when interpreted in this general model. The key is use of the same language and logic for the specification of programs as for description of the behavior of programs. We make no attempt to lift the level of abstraction above that of the atomic events involved in program execution.