 Although ‘levels of organization’ has been a key concept in biology and its philosophy since the early 20th century, there is still no consensus on the nature and significance of the concept. In different areas of philosophy and biology, we find strongly varying ideas of levels, and none of the accounts put forward has received wide acceptance. At the moment, the mechanistic approach is perhaps the most promising and acclaimed account, but as we have seen, it may be too minimalistic to fulfill the role that levels of organization continue to play in biological theorizing. We have also noted that there is a curious disconnect between the tradition ranging from Oppenheim and Putman to Bechtel, Craver and Wimsatt on the one hand, and the discussions of levels in evolutionary biology (e.g., Eldredge, McShea, Okasha) on the other. One important aim for future research would be to bridge this gap.