Many clinical pathologists and microbiologists will be called upon to decide whether to acquire a certain instrument or system available for microbiological analysis. What guidance do they need to make an intelligent decision? First, they need to understand the principle of action of the instrument and to evaluate how well this instrument fulfills its intended analytical function. The theoretical and practical considerations behind the design of instruments for microbiological analysis presented in the previous sections will aid the microbiologist-pathologist in understanding the requirements for an instrument system to perform these analyses. The following sections will discuss several approaches used in various sophisticated instrumented systems intended to perform one or more of the major phases of a microbiological analysis and can form the basis for the evaluation of acceptability of the instrument.