This paper reviews our present understanding of the energetics of extended defect ordering in modulated structures. We discuss the nature of extended defect interactions and describe theoretical models that have been developed to calculate the interaction energies. Continuum theories are considered first. Recently developed atomistic calculations are then discussed. We conclude with an account of the use of the atomistic approach in investigating a related problem, i.e. the factors which determine the energies of extended relative to point defect structures.