Due to the high complexity of modern-day business, organizations are forced to quickly adapt to a wide range of cutting-edge developments. These developments influence the structure and behavior of the business processes that represent the work and of the Business Process Management Systems (BPMS) that support them. Consequently, the architecture of BPMS has changed a lot over the past two decades. However, there is no systematic overview of the research done in this area since the Workflow reference model first set the standard for BPMS architecture in 1995. To bridge this gap, this paper presents a Systematic Literature Review (SLR) of BPMS architectures, by analyzing 41 primary studies taken from a gross collection of 608 research papers. The BPMS architectures that served as primary studies were compared with respect to the reference architecture that they are based on, the level of elaboration at which they are described, the architectural styles that they use, the means with which they are evaluated, and the functionality that they support. The resulting comparison provides an overview of and insights into the current body of knowledge on BPMS architectures.