Abstract
Penning ionization is a fundamenta atomic collision process whereby ions are produced in chemical reactions involving excited atoms. The process has been known since the 1920s but until fairly recently has been poorly understood. Over the past decade new experimental techniques such as metastable beams, flowing afterglows and Penning electron spectroscopy have produced a wealth of data on the process and have prompted the development of new theories. The process is now well described by the concept of an excited quasi-molecule which undergoes auto-ionization. In this article are described the recent experimental and theoretical work which has led to a new understanding of the process.