Very great changes have taken place recently in connection with rectifiers and their applications, and this seems to be a suitable moment to survey the state of world developments and the trends. The rectifiers considered here are limited to those of substantial power, for the ordinary industrial and traction applications. They therefore mainly comprise (i) mercury-arc convertors for wide-range grid control, and (ii) silicon rectifiers (with the thyristor growing in use), for electrochemical service, for 50c/s railway traction, and for nearly all other purposes. Among the subjects reviewed are the latest ideas on mercury-arc convertors, the methods of applying these to reversible drives, and the effects produced on the a.c. system. Rectifiers are now being increasingly used for field excitation of rotating machines; and improvements in variable-speed a.c. motors may be expected; The construction of semiconductor-diode cells is reviewed, as also are the methods of over-current and over-voltage protection. In the latter connection the `controlled avalanche rectifier' represents a promising new advance. Another very recent feature has been the combination of the transformer and rectifier as a single unit. The specially important field of the use of semiconductor rectifiers for 50c/s a.c. traction is briefly surveyed. Finally some account is given of the construction and characteristics of the thyristor, and of the circuit considerations that arise in its use.