Certain gut species are pathogens, but a number of other resident bacteria may be of some benefit to host health. Examples include enterococci, lactobacilli, propionibacteria, and bifidobacteria, which are present in the colon in significant numbers. Identification and classification are not identical. A group can be identified only after it has been classified, based on a pattern of properties shown by all the members of the group that other groups do not possess. The properties used in identification are often different from those used in classification. Biochemical, nutritional, and physiological characterization tests (usually carried out in bottles and tubes of solid and liquid media and on plates) have been developed and modified since the earliest days of bacteriology. Generally, the characteristics chosen for an identification plan should be easily determinable, whereas those used for classification (such as DNA homology) may be quite difficult to determine. Genera and species identification might not be based on only a few tests, but rather on the pattern given by a whole battery of tests. The members of the family Lactobacillaceae represent one example of this. Some probiotic strains can be selected for their benefical properties as active antimicrobial agents against pathogenic microorganisms, hydrophobic ability, presence of substances with a capacity for adherence to epithelium, and so on. After isolation, identification is an important step before selecting probiotic strains. These identifications can be facilitated through microtechniques. To alleviate the need to inoculate large numbers of tubes with media (conventional test), some rapid multitest systems have been devised and are commercially available (such as the API or Biolog systems). Although they are expensive for large studies and not always sufficiently versatile, these kits do offer the advantages of convenience, miniaturization, rapidity, and, above all, strict standardization. Other methods such as ribotyping or randomly amplified polymorphic DNA do not take into account the phenotypic characteristics (biotyping). In this chapter, we describe a simple, rapid, and economical microplates technique to identify lactic acid bacteria and enterobacteria based on general metabolic characteristics, adapted for routine study of a large number of strains.