Defining soft law is a difficult task as no real consensus has emerged in the scholarship on the criteria that could help to delimit its contours. Considering the diversity of existing definitions, this chapter delineate soft law by using two types of characteristics. Firstly, soft law is situated in between hard law and non-law. Some elements are missing for soft law to be considered (proper) hard law. On the other hand, resemblance with hard law is what makes soft law differ from non-law. Indeed, a soft law act is formalized in a way that makes it close to a hard instrument, and it aims at influencing, modifying, and directing the behavior of their addressees. Secondly, soft law is further defined by looking at the function it performs, which is usually done through an analysis of their interactions with hard law. Soft law may anticipate hard law (pre-law), follow the adoption of hard law (post-law), or be considered as a substitute to hard law (para-law). Finally, the chapter argues that a more detailed definition could help to better analyze and understand the use of soft law at domestic, EU and international level.