The Flight Software (FSW) for the Hubble Space Telescope (HST) advanced camera for surveys (ACS) is complex, real- time, embedded software system. It is responsible for non- board command and telemetry processing, instrument maintenance, detector and mechanism control, execution of scientific observations and science data collection - in other words, control of over 40,000,000 dollars of hardware which is beyond the physical grasp of the ground system. To ensure reliable and expected performance, it is subject to an extensive test and integration process. The test program has matured from its first use during second generation near-IR camera multi-object spectrometer and space telescope imaging spectrograph FSW development, to a proven methodology currently used on ACS, a third generation instrument. The test philosophy has its foundation in solid software development practices, and most importantly, focuses on lengthy testing with high-fidelity flight and ground operations simulations and early integration with flight hardware. This paper will describe the test and integration process for the ACS FSW, with particular emphasis on the expectations at each level and the program resources required to meet those goals. We will provide quantitative measures of the benefits of the approach, using actual examples from ACS FSW test and integration. Finally, the paper will highlight the lessons learned and provide ideas which may be applied to future flight software development efforts.