TRANSISTORS WERE INVENTED 50 YEARS AGO next week. This simple three-terminal electronic device can act as amplifier or switch by allowing a tiny electrical (gate) signal to control a much bigger current. (For the history of the transistor, see the December Physics Today and the book "Crystal Fire: the Birth of the Information Age," by Michael Riordan and Lillian Hoddeson.) Examples of ongoing research include the development of all- polymer transistors (Update 196); spin transistors, in which the spin as well as the charge of electrons is important (Physics Today, July 1995); room-temperature, single-electron transistors (Update 308); silicon-carbide transistors for high-temperature applications (Update 327); 10-nm metal transistors (Update 322); the development of molecular-scale transistors (New Scientist, 2 Aug 1997); and neuron transistors, in which gate signals are supplied by ions from leech neurons (Jenkner and Fromherz, Phys. Rev. Lett., 8 Dec.)