Despite some dismissal of the research value of its idiosyncratic materials, a substantial body of scholarly publications now uses first-phase Mass Observation papers (1937–50s). An emerging body of literature also draws upon the post-1981 papers of the Mass Observation Project. While Mass Observation’s significance as a historical source is demonstrable through its vast scale, unique holdings and acknowledged international standing, how the eclectic form and content of such materials might best be interpreted is less clear. This article provides a summary of and engagement with the issues of methodological concern to researchers using the archival materials generated by the Mass Observation Project, in particular at the point of its thirtieth anniversary, and also reflects on methods of historical interpretation more generally. Core debates considered include the complex legacies of the original organization and ongoing concern with the demographic ‘representativeness’ of the volunteer panel. The sometimes challenging form and scale of the archival material, and the always interdisciplinary nature of the project (and its researchers), lead to a range of differences in how the material is characterized, as well as in the sampling and interpretive models employed. Through an exploration of debates raised in the recent University of Brighton research network Methodological Innovation: Using Mass Observation, alongside a review of publications that reflect, methodologically, on the use and value of the Mass Observation Project, this article evaluates varied and cross-disciplinary research practices and approaches to present a defence of Mass Observation material as a unique means of accessing the difficulties and complexities of messy everyday life.