This review of process analytical chemistry is a continuation of the series of reviews on this subject published in 1999, 1995, and 1993. This review covers the period from 1999 through March 2001. Many changes have occurred in this field due to the obvious rapid advances in measurement and computer technology. These advances are a direct result of specialized research aimed at solving both fundamental and applied problems encountered in real-time measurement and process control of chemical and biochemical phenomena. This review, in a more condensed format than in the past, addresses the basic measurement disciplines inclusive within process analytical chemistry with the addition of microanalytical systems, as well as X-ray and ç-ray spectrometry. The 1999 review (1), covering the period from 1995 through 1999 with 1301 references, is the basis for the topics covered within this review. Detail is given for some research papers that encompass the most recent developments in measurement techniques over the past two years.