A computer-programming methodology that focuses on data items rather than processes. Traditional software development models assume a top-down approach. A functional description of a system is produced and then refined until a running implementation is achieved. The refinement process breaks each system function down into subfunctions which combine to implement the higher-level function. The emphasis of the approach is on the transformation of inputs to outputs. Data structures (and file structures) are proposed and evaluated based on how well they support the functional models.