Real-time particle deposition monitoring is a new way to monitor the operational quality of a cleanroom. Up to now the performance of a cleanroom is often monitored with light-scattering airborne particle counters. These give valuable information on airborne particles, but no information on the potentially threatening macroparticles in a cleanroom. In this paper, particle deposition phenomena and ways to determine particle deposition are discussed. A real-time particle deposition monitor using holographic imaging of the witness plates in a sensing device is described. Some practical experiences with real-time particle deposition measurements are demonstrated. Real-time particle deposition monitoring can be a valuable method to control the operational quality of a cleanroom where personnel are working.