Publisher Summary In order to understand the functions of microbes in nature, as opposed to the task microbes are capable of doing in the laboratory, ecologists must make measurements of rates of microbial processes in the real world. One obvious and important process is metabolism and growth of bacteria on organic compounds supplied from outside the cell. The techniques discussed in the chapter have given tremendous amount of information about the role, rates of activity, and factors limiting the heterotrophic bacteria in the plankton. Only some of the methods, those using radioisotopes, are presented in detail in the chapter. These are all variations on the theme of the addition of extremely low concentrations of labelled organic substrates to water samples. Two kinds of measurements have been made, relative and absolute. Absolute measures give true rates of microbial metabolism and growth, while relative measures give values that are positively correlated with the true rates but are not identical.