Space charge as it affects ion beam transport is reviewed. The approach here will be to derive beam-current criteria for divergence from space charge, review recent theoretical models for fractional space-charge neutralization, discuss space-charge-related observations on ion-beam transport in a specific experimental system, and briefly note several applications using space charge. Experimental measurements of effective space charge are discussed for a dc ion-source test stand using a 90/sup 0/ double-focusing magnet for species separation and for a solenoidal lens magnet for trim focus of the ion beam preparatory to entrance into a 400-kV accelerator column.