Mixed methods are methodologies that focus on collecting, analysing, and mixing quantitative and qualitative data in a single study or series of studies. Although they may be employed in any form of scientific inquiry, mixed methods designs are especially appropriate for use in implementation science due to the complexity of the phenomena under investigation and the opportunity to examine different aspects of these phenomena either in sequence or simultaneously. This essay describes the various decisions required for conducting a mixed method investigation, including the weight given to each method, the timing of their use, the process of integrating quantitative and qualitative data and analysis, and the function of the integration. Quantitative and qualitative data may be used to answer the same question through convergence or transformation or to answer related questions through complementarity of focus (e.g., breadth versus depth of understanding, implementation process versus outcomes); expansion or explanation of findings obtained from one method by use of the other method, development of measures, frameworks, or hypotheses; or sampling of study participants.