An overview is presented of the present status of on-line measuring methods for analysis of metal species, particularly alkali and heavy metals, in process gas. The motivation for developing such techniques is both environmental and economic, therefore a summary of the chief issues of concern is given first. Two broad groups of techniques are reviewed: optical methods (absorption, spontaneous emission and induced fluorescence) and ionisation methods, including mass spectrometry. Many of the more recent techniques involve the use of laser technology. The main features of the methods are presented first together with a description of the problems encountered in quantifying measured data. An intermediate section then briefly describes sampling methods. Finally, those measuring techniques that have proved to be amenable to realistic process conditions are considered again in the applications section. Their progress and capabilities to date are illustrated by a representative selection of results obtained in a variety of combustion and incineration facilities: (C) 2002 Published by Elsevier Science Ltd.