  We introduce a rigorous framework for the quantification of coherence and
identify intuitive and easily computable measures of coherence. We achieve this
by adopting the viewpoint of coherence as a physical resource. By determining
defining conditions for measures of coherence we identify classes of
functionals that satisfy these conditions and other, at first glance natural
quantities, that do not qualify as coherence measure. We conclude with an
outline of the questions that remain to be answered to complete the theory of
coherence as a resource.
