Parsons problems are a type of programming exercise where students rearrange jumbled code blocks of a solution program back into its original form. It is usually implemented as a complement or alternative to traditional programming exercises like code-tracing and code-writing. This paper reviews the existing literature on the Parsons problem in introductory CS education. We find that the flexible nature of the design of Parsons problems has led to many variants, and these have been continuously refined to better address student needs. However, the effectiveness of Parsons problems, both as a question type and as a learning tool in CS education, remains uncertain due to a lack of replicated research in the field.