Tandem‐accelerator operation requires a source of negative ions. The earliest negative‐ion sources were based on charge exchange of positive ions from a rf or duoplasmatron source. The development of the sputter‐ion source in the 1970’s produced a revolutionary change in the number of negative‐ion beams for Tandem operation. This article will review operational characteristics of sputter‐ion sources coupled to large Tandem accelerators. These characteristics and matching of the ion source to the accelerator are determining factors for the production of useful beam intensities for physics experiments. A new approach using a 2.45 GHz microwave ion source to produce a high‐quality, positive‐ion beam coupled to a charge‐exchange canal is being developed at Chalk River Laboratories, and some results of this development program will be presented. Plans for future development include the investigation of direct extraction of negative ions from a microwave‐driven plasma. ©1996 American Institute of Physics.