chemical sensing technologies impinge upon a range of scientific disciplines. there is an overwhelming need for simple detection and monitoring devices suitable for use by the non-specialist analyst, particularly in the field. this review outlines the basic principles of the major chemical and bio-sensing technologies. these include ion-selective electrodes for inorganic ions, polarographic devices for o//2 and redox species, as well as enzyme-coated devices for general substrate measurement based upon thermal or electrochemical principles. alternative use of fiberoptic probes and piezoelectric sensors is also indicated. irrespective of basic chemical principles, a fundamental concern with all such systems is the need for optimised interfacing with the sample matrix inevitably, this necessitates specific and special manipulation of both the surface properties of devices and the access given to analyte, interferent and potentially fouling species present in solution. the review, thereby, highlights the fact that optimised covering membranes are an important functional element in sensors, and one which needs the concerted action of membrane scientists (author abstract) 25 refs