  We briefly review some common diffusion-limited reactions with emphasis on
results for two-species reactions with anisotropic hopping. Our review also
covers single-species reactions. The scope is that of providing reference and
general discussion rather than details of methods and results. Recent exact
results for a two-species model with anisotropic hopping and with `sticky'
interaction of like particles, obtained by a novel method which allows exact
solution of certain single-species and two-species reactions, are discussed.
