By enabling real time monitoring and control of particle levels in integrated circuit process equipment, in situ particle monitors (ISPMs) have the potential to reduce dramatically the dominant source of yield-killing defects, i.e., process induced particles. However, there are also significant limitations to their use for on-line monitoring. A survey of the ISPM literature shows that the greatest benefit provided by ISPMs for most applications is their immediate indication of high-particle excursions. In only a few applications described in the literature do the sensor data correlate with wafer surface scans and show enough sensitivity to baseline particle levels to control process maintenance cycles. Scaling arguments show that at gas pressures below about 100 mTorr, gravitational settling and particle inertia typically prevent particles from being carried efficiently to exhaust sensors, in spite of experimental evidence that bouncing significantly aids particle transport. Combined with the low sampling efficiency of most sensors, these limitations make exhaust sensors impractical for many applications including most sputtering and many etching processes, unless chambers are regularly vented to high pressures. The cutoff imposed by particle transport limitations is clearly demarcated in the ISPM literature, which indicates good sensitivity of exhaust sensors to baseline particle levels in high pressure processes such as plasma-enhanced chemical-vapor deposition, sensitivity only to high-particle excursions for processes around 100 mTorr, and insensitivity to particle levels in processes that operate at low pressures, except during gas venting or purging. For many applications, standard sensors are susceptible to false counting due to fouled optics, noise from rf and magnetic fields, and optical emissions. High current ion implanters, with their nearly noise-free chambers, are the only fully proven application of in-chamber sensors. To overcome the particle transport limitations in other critical defect-producing processes that operate at low pressure, in-chamber sensors that are insensitive to the harsh chemistry and high noise levels of process chambers need to be developed.