The limitations of traditional forms of systematic review in making optimal use of all forms of evidence are increasingly evident, especially for policy-makers and practitioners. There is an urgent need for robust ways of incorporating qualitative evidence into systematic reviews. In this paper we provide a brief overview and critique of a selection of strategies for synthesising qualitative and quantitative evidence, ranging from techniques that are largely qualitative and interpretive through to techniques that are largely quantitative and integrative. A range of methods is available for synthesising diverse forms of evidence. These include narrative summary, thematic analysis, grounded theory, meta-ethnography, meta-study, realist synthesis, Miles and Huberman's data analysis techniques, content analysis, case survey, qualitative comparative analysis and Bayesian meta-analysis. Methods vary in their strengths and weaknesses, ability to deal with qualitative and quantitative forms of evidence, and type of question for which they are most suitable. We identify a number of procedural, conceptual and theoretical issues that need to be addressed in moving forward with this area, and emphasise the need for existing techniques to be evaluated and modified, rather than inventing new approaches.