Since the notion of naturalness was first used for nature conservation, it has been used quite often for forest ecosystems. The multitude of terms used to characterise "natural" forests reflects how complex the concept is. In this article, we propose to analyse the relationships between long time unmanaged forests and naturalness, and to distinguish between the two main types of naturalness, namely biological naturalness and anthropogenic naturalness — anthropogenic naturalness including the notions of absence of management or of process spontaneity. Then, we propose a summary of the metrics used in the scientific literature to measure these two types of naturalness in forests. The metrics linked to biological naturalness are more focused on aged and senescent stages, while the metrics linked to anthropogenic naturalness are more focused on past wood cuts (the time elapsed since the last cutting campaign, logging intensity). Finally, we discuss these notions within a wider context – not just a forestry context. This leads us to discuss the biases related to the choice of the metrics, the link between biodiversity and naturalness, the reference state in a global change context, and the relationship between long time unmanaged forests and rewilding.