Whole cells that contain enzyme(s) if interest for particular biorecognition events are a very attractive alternative to the use of purified enzymes. Their inexpensiveness recommends them in particular as catalysts in redox reactions requiring a cofactor-dependent enzyme. Yeast cells provide both enzyme and cofactors and this makes for a simple experimental approach. The review presents applications of yeasts in biosensing for clinical laboratory, environmental field, toxicity and genotoxicity, and the food industry. Engineered yeasts are also introduced as potential bioactive elements in sensors and discussed in comparison with yeasts from natural sources and yeast extracts. Different types of transducers are presented in a separate chapter.