A variety of techniques are currently in use for process monitoring and control of wafer quality during production. In general these techniques fall into two categories: (1) particle monitors that provide fast, simple results but are limited in visibility to many defects, and (2) more advanced systems that monitor a much larger class of potential problems but require significant analysis time and interpretation of results. The first category has traditionally been used for in-line monitoring, while the second category has served primarily for engineering analysis and R&D applications. Recent technical developments, in particular the development of advanced systems based on optical pattern filtering, have begun to blur the distinction between in-line and engineering analysis tools. This paper establishes performance guidelines for in-line process monitoring tools, reviews the current techniques utilized against these guidelines, and discusses the potential applications of these methods to in-line process control. Systems based on these new techniques hold the promise of providing sophisticated analysis capability for in-line process control applications, by offering extremely high throughput (in the range of a few minutes per wafer) and high sensitivity (0.20 micrometers and better), combined with intelligent but fully automated analysis of the data to provide `single-number' reports on production wafers.