Two of the most sensiive methods currently available for monitoring membrane integrity are the pressure decay test (PDT) and diffusive air flaw (DAF) tests. Both these tests are capable of monitoring small changes in membrane integrity, even in very large membrane arrays. The DAF test, for example, is capable of detecting a single broken fibre in a membrane array containing over one million fibres. This enables operators to control membrane integrity at levels well above 4 log removal of Giardia or Cryptosporidium, without relying on filtered water quality monitors such as turbidity or particle counters. The tests form part of an overall membrane control strategy known as the Memsure(TM) process, involving process integrity monitoring, and identification and isolation of faulty modules for later repair. Warren T. Johnson, Memtec Research Pty Ltd, Australia delivers an overview of this process.