Bioassays are useful tools in modern biosciences. The applications include development of new drugs; measurement of biological activity in undefined substances; investigation of endogenous mediator function; determination of drug toxicity; and measurement of the concentration of known substances such as pollutants or herbal products. As medicinal plants are very popular on market, the safety and/or quality control of these phytotherapeutic agents become an important issue. Currently, botanical identification, molecular identification and chemical analysis are often utilized for the quality analysis of plant products. However, the methods are not totally reliable due to factors such as seasonal and geographical variations in plant growth. An important development of herbal product quality analysis has been the use of biological assay systems which can test many samples for activity in a short period of time. The advantages of using bioassays in this aspect are many-fold. The tests can avoid the ethical and financial constraints of using human subjects or larger animals. Bioassay-guided isolation of ‘active’ compounds can be achieved and much smaller amount of test material is needed. However, it is important to realize that there is no single method to assure absolute quality. Bioassays combined with plant macro and microscopic data, chemical profiling and DNA fingerprinting would probably offer the safest and most efficacious herbal products.