The main theme in Volume 6 of “Methods in Microbiology” is the application of biochemical techniques to the study of micro-organisms. The topics covered include the use of biochemical and enzymic tests to charac- terize microbial types, the quantitative separation and analysis of fermenta- tion products produced by micro-organisms, a critical appraisal of methods available to elucidate metabolic pathways and the control of enzyme syn- thesis, the assay of selected enzymes in crude extracts of cells as indicators of metabolic pathways, the use of antimetabolites to study biosynthesis and electron transport. The applications of radiotracer techniques are described in detail in five chapters: respiratory measurements with dyes and with electrodes for oxygen and carbon dioxide are included while manometric methods, which are so well described elsewhere (Umbreit, Burris and Stauffer, 1964) have been excluded from this series; articles on nitrogen fixation, calorimetry, electrochemical measurements and methods for studying sporulation and germination are also provided.