  Infrared safe differential cross sections, such as event shape distributions,
can be measured over wide kinematic ranges, from regions where fixed order
calculations are adequate to regions where nonperturbative dynamics dominate.
Such observables provide an ideal laboratory for the study of the transition
between weak and strong coupling in quantum field theory. This talk begins with
some of the fundamentals of the perturbative description of QCD and the basis
of resummation techniques, followed by a brief discussion of selected topics
from recent fixed-order and resummed calculations. It focuses on how resummed
perturbation theory has been used to deduce the structure of nonperturbative
corrections, and to provide a framework with which to address the transition
from short- to long-distance dynamics in QCD.
