This chapter reflects on how evidence from realist trials and systematic reviews might be of value, not only in drawing conclusions about specific interventions and their theories of change but also in testing and refining the middle range theories which inform these and other interventions. While evaluation evidence should be of most immediate use in informing decisions about the implementation of the specific interventions being evaluated, a broader and more enduring use for evaluation could be in suggesting refinements to middle range theory. Such refinements might then be used to inform and influence the next generation of complex health interventions. In order to be useful in assessing the validity of middle range theory, evaluations will need to assess interventions informed by a limited number of middle range theories comprising a limited number of well-defined constructs. There may be value in conducting proof of principle studies separately from more pragmatic evaluations in order to test and refine middle range theory.