Use of a radioisotope power source coupled to a thermoelectric converter represents one attractive technique for filling the need for a long-life reliable electrical power supply in the range from 100 to 1000 mW. Reliable supplies that require no maintenance for 5 years are available. Existing systems weigh 400 to 850 lb and cost $12 000 to $15 000/W(e). Systems weighing <10 lb and costing $20 000 to $27 000 can be built using existing designs and technology. The radioisotope power supply is attractive for underwater power and other specialized applications where reliability, long life, low weight, or compactness is more important than cost. Radioisotope thermoelectric generators in this size range provide power from ≈$0.27 to $0.70/(Wh), depending upon the design. The design characteristics of five typical generators are described. They include 1-Wdesigns for oceanographic applications and a 300-mW design for an ordnance application. One generator is commercially available, while others are concepts. Characteristics of several other available generators are summarized.