Meta-analysis comprises the analysis of the data of similar studies gathered in a systematic review. If the results of the studies differ (strongly), this is called heterogeneity. Possible causes of heterogeneity are: a wrong choice of type of measure of treatment effect, differences of methodological quality between studies, or real differences between studies. Heterogeneity between studies can be assessed by thorough examination of the differences between study characteristics in combination with a visual inspection of the degree of overlap of the confidence intervals of the estimates of effect of the different studies. By applying the random effects model for pooling, (non-systematic) heterogeneity between studies can be addressed. In case of heterogeneity between studies, sources of heterogeneity should be explored by means of subgroup analysis according to strict, present criteria. Subgroup analysis should address subgroups of patients within studies.