In Webster’s New International Dictionary, Second Edition (1930), “sentinel” is defined as “One who watches or guards . . . from surprise, . . . observe(s) and give(s) notice of danger.” This definition of sentinel is carried over into the two concepts of sentinel species as organisms and sentinel bioassays as measurable changes in biological functions, both of particular relevance as biological indicators of environmental contamination. The use of both sentinel species and sentinel bioassays is germane to laboratory and field analyses and to assessing the human and environmental health effects of toxic substances and hazardous conditions. The two concepts can be applied independently or together, but the combined use of sentinel species and sentinel bioassay is probably the most effective. The ideal application of sentinel is as an early warning system, in keeping with the dictionary definition. In this sense it is a “biological radar” warning of approaching hostile conditions.