The interest of lean companies in tools to monitor and control their lean implementation process, i.e. their leanness level, is continuously increasing. A multitude of diverse approaches have been proposed in the literature for that purpose. In order to provide some guidance both to practitioners interested in selecting a tool and academics aiming at developing a new approach, a systematic review of the methods to measure leanness in manufacturing organisations is carried out. Thirty-one methods are identified and critically analysed based on a set of comparative dimensions. The main strengths and weaknesses of each approach are highlighted. In particular, this paper's results show that only less than a third of the methods are able to assess leanness in a complete way, taking into consideration both the degree of lean practices adoption and the performance outcomes resulting from implementation. In order to contribute to the reinforcement of a unifying view of the leanness concept, a new leanness definition is developed. Some further gaps in the literature are pointed out, providing directions for future research.