A particularly appealing quantum-chemical subsystem approach is provided by subsystem density-functional theory (sDFT). It achieves a partitioning in terms of the electron density, which is a measurable real-space quantity, and thus provides a chemically intuitive and versatile framework. Since the publication of our original review on sDFT in 2014 , the body of scientific literature either using sDFT and related methods or exploring new methodological avenues along the lines of sDFT has increased tremendously. In this Update, we provide the readership with a review of these developments and illustrate the potential of applications that have emerged in the meantime. Subsystem density-functional theory is closely tied to the concept of density-based embedding and to theories such as Frozen-Density Embedding (FDE) . Over the past years, several similar methods have emerged, which can be regarded as either technically or conceptually related. And even the formerly sharp boundary between mixed quantum chemical/classical (QM/MM) methods and density-based embedding methods of QM/QM type is fading. Thus, while our original article focussed on the fundamental and applications of sDFT itself, in this Update we will broaden our perspective and also discuss related approaches in the context of sDFT.