This review covers approximately the literature reported in Chemical Abstracts over the period January 1974 to December 1975, plus some additions from other sources. As the editors have requested, emphasis is on developments in theory, methodology, and instrumentation, with applications included only insofar as they appear to advance the state-of-the-art or have particular current relevance. The selection of what to include necessarily is a matter of judgment, and so certain areas may not have received the attention considered appropriate by some. The comments and suggestions of readers are welcomed.