Service-Oriented Architecture (SOA) is a new architectural style for developing distributed business applications. Nowadays, those applications are realized through web services, which are later grouped as web service compositions. Web service compositions language, like the BPELWS 2.0 standard, are extensions of imperative programming languages. Additionally, it presents a challenge for traditional white-box testing, due to its inclusion of specific instructions, concurrency, fault compensation and dynamic service discovery and invocation. In fact, there is a lack of unit testing approaches and tools, which has resulted in inefficient practices in testing and debugging of automated business processes. Therefore, we performed a systematic review study, to analyze 27 different studies for unit testing approaches for BPEL. This paper aims to focus on a comprehensive review to identify a categorization, a description of test case generation approaches, empirical evidence, current trends in BPEL studies, and finally to end with future work for other researchers.