Investigating assembly complexity is a continuing concern within manufacturing field, as it may impact quality and costs of products. There is a growing body of literature aimed at developing novel and reliable methods to assess assembly complexity. The first aim of this paper is to provide an extensive literature review, identifying the main approaches to this problem. Three main approaches emerged: product-centred approach, information-centred approach and system-centred approach. A second goal is to carry out a comparative analysis of these approaches highlighting their advantages, limitations and providing guidance for process and product designers seeking to control assembly complexity. For each approach, a representative method was chosen, analysed in detail, and applied to three different real products.