Theory and phenomenology of non-global logarithms

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.