The demand and requirements for microprocessor based on-board test systems are growing. This paper highlights the versatile role of a building block approach called Integrated Data Acquisition and Control System (IDACS) for designing and implementing customized in-vehicle test systems. With the IDACS approach each system is partitioned into hardware and software modules which can be purchased or designed as building blocks with customized interface to the specific application. Commercially available hardware subsystems are used and connected through an industry standard bus. IDACS software modules are designed to provide extensibility, pliability and consistency across the different applications. IDACS software is modular, portable, totally table driven and written in the high level language C. A generalized multitasking design based on a real-time executive has enabled it to be adapted for a number of on-board test systems. Details of the approach used in developing IDACS hardware and software are described. Examples of its utilization for on-board testing are presented.