Among researchers' many investigations of the use of mixed methods in intervention studies, more recent discussions especially concern the roles played in such studies by qualitative research, intervention phases, procedures, and integration (Gallo and Lee, 2016; Woolcock, 2018; O'Cathain, 2018; Creswell and Plano Clark, 2018). One can find the basic procedures to follow while realizing a mixed methods experimental design (Creswell and Plano Clark, 2018), practical guidance (O'Cathain, 2018) for using qualitative research with a randomized control trial (RCT), and a mixed methods appraisal tool for appraising the methodological quality of RCTs, non-randomized studies, and mixed methods – MMAT (Hong et al., 2018). However, no model exists to assess the quality of mixed methods research in intervention studies, particularly experimental and quasi-experimental research in complex interventions. Our aim is to develop such a theoretical model. Today, the number of interventions relying on mixed methods methodology is growing exponentially. A theoretical model is called for to help assess the quality of mixed methods research in intervention studies, and in this respect our aim is to: (1) provide an overview of guidelines, recommendations, models, and quality criteria for mixed methods research; (2) overview the guidelines for intervention studies; (3) give a summary of guidelines and models for mixed methods research in such studies; (4) evaluate the mentioned guidelines, models, and quality criteria; (5) identify and describe the key elements of these guidelines, models, and quality criteria; and (6) develop a theoretical model for assessing the quality of mixed methods research designs used in intervention studies.