Intense ion beams are an extreme example of, and difficult to maintain as, a non-neutral plasma. Experiments and simulations are used to study the complex interactions between beam ions and (unwanted) electrons. Such "electron clouds" limit the performance of many accelerators. To characterize electron clouds, a number of parameters are measured including: total and local electron production and loss for each of three major sources, beam potential versus time, electron line-charge density, and gas pressure within the beam. Electron control methods include surface treatments to reduce electron and gas emission, and techniques to remove, or block, electrons from the beam. Detailed, selfconsistent simulations include beam-transport fields, and electron and gas generation and transport, to compute unexpectedly rich behavior, much of which is confirmed experimentally. For example, in a quadrupole magnetic field, ion and dense electron plasmas interact to produce multi-kV oscillations in the electron plasma and distortions of the beam velocity space distribution, without becoming homogenous or locally neutral.[159 words, ~150 allowed] 2