  We discuss the theoretical treatment of non-global observables, those
quantities that are sensitive only to radiation in a restricted region of phase
space, and describe how large `non-global' logarithms arise when we veto the
energy flowing into the restricted region. The phenomenological impact of
non-global logarithms is then discussed, drawing on examples from event shapes
in DIS and energy-flow observables in 2-jet systems. We then describe
techniques to reduce the numerical importance of non-global logarithms, looking
at clustering algorithms in energy flow observables and the study of associated
distribution of multiple observables.
