Over the last decade, many healthcare organizations have adopted lean thinking to reduce wastages and to improve flow efficiency in the patient flow processes. But there is limited research done on understanding the sequence to be followed by healthcare organizations to implement lean in their different departments, and on the applicability of various lean tools in different healthcare settings. This article aims to bridge this gap, map the current state of lean in healthcare and identify future research directions. Key concepts of lean thinking are introduced, and the current body of knowledge between 2002 and 2014 is explored to answer key research questions on the application of lean in different healthcare settings. Further, this article is one of the first attempts to classify healthcare services according to the service process matrix. Results show that different healthcare services pose unique challenges to lean implementation due to their inherent characteristics of service delivery. Research opportunities exist to determine the appropriate path which a hospital should follow while implementing lean, to identify the most appropriate set of lean tools and for empirically validating the benefits of lean.