Design methods capture key procedural knowledge, central to design process, practice, and education. However, a more robust explanation of the method phenomenon is needed. We move towards answering this need by proposing, and quantitatively testing, a Method Content Theory; addressing i) how methods function in context; ii) the elements of method content i.e. Method Framing, Method Rationale, Method Goal, Method Procedure, and Method Mindset; and iii) predictors of method performance. We provide initial quantitative validation for the proposed theory demonstrating strong predictive power for methodological performance indicators. We build on this to define a future research agenda. Our results have implications for research, education and practice, along with the potential for actionable insights in design methodology, method development and validation.