Cosmic ray transient variations are signatures of the underlying solar
processes which affect the heliomagnetic structure. They can be found
in the data from surface systems, such as neutron monitors and muon t
elescopes, and from shallow underground muon telescopes. Although some
times observable at all latitudes, the global distribution of the effe
cts is important in determining the structures and causes. Three trans
ient variation types are known, namely Forbush decreases, ground level
enhancements, and quasi-periodic fluctuations. The latter category in
cludes some variations which are, perhaps, too long-lived to be consid
ered truly transient. In this review, the detection techniques and bac
kground of cosmic ray research axe followed by a summary of some obser
vations in each transient category. The heliomagnetic structures and d
ynamics inferred from such transients are discussed together with the
possible impact such events can have on human activity.