"n "n "nAbstract: "nIn contrast to traditional reviews, systematic reviews explore theliterature on a specific topic comprehensively using a well-defined protocol and recruit the findings of eligible studies after the assessment of their qualities, using a clear and reproducible method. Checking the assumptions, we may merge the findings of recruited studies in a systematic review and use metaanalysis techniques not only to estimate the pooled effect but also to assess the possible sources of heterogeneity and the extend of heterogeneity in the findings . Generally, randomized clinical trials are the most common type of studies which are recruiting in systematic reviews and meta-analyses. However, we couldn't ignore the importance of observational studies particularly in sensitivity topics with ethical limitations against interventional studies. Although we are usually faced with wide variations in the methodology of observational studies even in narrow topics, systematic review and meta-analysis of their results can generate valuable findings. They are particularly useful for explaining the source of heterogeneity in the results of primary observational studies .In this paper, we review the basic concepts of the systematic review and metaanalysis and their main applications in summarizing the findings of observational studies with respect to their advantages and limitations.