The success of modernizing legacy software systems to Service-Oriented Architecture (SOA) depends on Service Identification Approaches (SIAs), which identify reusable functionalities that could become services. The literature describes several SIAs. However, the selection of an identification approach that is suitable for a practitioner is difficult because it depends on several factors, including the goal of modernization, the available legacy artifacts, the organization’s development process, the desired output, and the usability of the approach. Accordingly, to select a suitable service identification approach, a practitioner must have a comprehensive view of existing techniques.We report a systematic literature review (SLR) that covers 41 SIAs based on software-systems analyses. Based on this SLR, we create a taxonomy of SIAs and build a multi-layer classification of existing identification approaches. We start from a high-level classification based on the used inputs, the applied processes, the given outputs, and the usability of the SIAs. We then divide each category into a fine-grained taxonomy that helps practitioners in selecting a suitable approach for identifying services in legacy software systems. We build our SLR based on our experience with legacy software modernization, on discussions and experiences working with industrial partners, and analyses of existing SIAs. We validate the correctness and the coverage of our review with industrial experts who modernize(d) legacy software systems to SOA. The results show that our classification conforms to the industrial experts’ experiences. We also show that most of the studied SIAs are still at their infancy. Finally, we identify the main challenges that SIAs need to address, to improve their quality.