The matrix isolation (MI) method has now been used for nearly thirty years. During this period it has been actively developed and the range of problems tackled greatly extended. Originally it was used for studies of transient species involv- ing vibrational, electronic and ESR spectroscopy. Nowadays the study of transient species forms a comparatively small part of HI work since it has been amply demonstrated that very fruitful information can be obtained of the structure and interactions of stable molecules and their aggregates. In addition to the s ectroscopic methods mentioned above the MI...