  We describe the use of generalized unitarity for the construction of
correlation functions of local gauge-invariant operators in general quantum
field theories and illustrate this method with several calculations in N=4
super-Yang-Mills theory involving BPS and non-BPS operators. Form factors of
gauge-invariant operators and their multi-operator generalization play an
important role in our construction. We discuss various symmetries of the
momentum space presentation of correlation functions, which is natural in this
framework and give examples involving non-BPS and any number of BPS operators.
We also discuss the calculation of correlators describing the energy flow in
scattering processes as well as the construction of the effective action of a
background gravitational field.
