Transistors may be used in both d.c. and a.c. voltage stabilizers and their use is considered in both applications although emphasis is placed on their use in a.c. voltage stabilizers. The operating conditions of transistors in a.c. stabilizers are different from their normal operating conditions in amplifiers and care is necessary to prevent damage to them. The operation of a transistor essentially as a variable resistor is considered. Transistors fed from auxiliary sources of power (either d.c. or a.c.) may absorb power or give out power and a number of circuits are considered including those using a complementary pair.