Two in-situ testing methods developed by Memcor have been shown to provide a sensitive measure of system integrity for microfiltration membranes. These are the pressure decay test (PDT) and diffusive air now test (DAF). The tests enable operators to measure the log removal capability of membrane systems directly, rather than relying solely on filtered water quality monitors such as turbidity and particle counters. The DAF test, for example, is capable of detecting a single broken fibre in a membrane array containing over one million fibres. This paper discusses the principles and application of the PDT and DAF methods to the monitoring and control of membrane system integrity.