  In exchange processes clusters composed of elementary building blocks,
monomers, undergo binary exchange in which a monomer is transferred from one
cluster to another. In assortative exchange only clusters with comparable
masses participate in exchange events. We study maximally assortative exchange
processes in which only clusters of equal masses can exchange monomers. A
mean-field framework based on rate equations is appropriate for spatially
homogeneous systems in sufficiently high spatial dimension. For
diffusion-controlled exchange processes, the mean-field approach is erroneous
when the spatial dimension is smaller than critical; we analyze such systems
using scaling and heuristic arguments. Apart from infinite-cluster systems we
explore the fate of finite systems and study maximally assortative exchange
processes driven by a localized input.
