 We have focused on core domains of cell biology, concerned with explaining the basic activities of cells. There are, though, many specialized fields concerned with cells. Some of these have become active areas for philosophical inquiry in their own right. Here we merely identify some of these and point readers to relevant work by philosophers, including in several cases entries in the SEP. One such domain is microbiology, concerned with single-cell organisms, whether prokaryotic (lacking a nucleus and other organelles) or eukaryotic. O’Malley (2014) has identified distinctive features of microorganisms (e.g., lateral gene transfer) and explores how features of microorganismal life challenge major assumptions about living organisms that have resulted from focusing predominantly on multi-cellular organisms. In other work (O’Malley 2010), she has examined from a philosophical perspective the competing hypotheses about the origin of eukaryotic cells from prokaryotic cells. Much of cell biology has focused on mature cells of specific types and has attended less to the processes by which cells in multi-cellular organisms differentiate from a common cell, known as a stem cell. Fagan (2013) has pioneered the philosophical examination of stem-cell research, including the advent of techniques to revert mature cells to stem cells. Cells are not only transformed in developmental processes in multi-cell organisms, but also in diseases such as cancer in which cells not only proliferate in uncontrolled ways but also defeat many normally operating cellular mechanisms that normally prevent proliferation. Plutynski (2018) has identified many of the philosophical challenges arising in the efforts to explain cancer (see the entry on cancer). An important capability of many multicell organisms is the ability to detect pathogens, viruses, and other threats and defend against them. Such immune responses require, among other things, the capacity to distinguish cells that belong to the organism itself from others (see the entry on  philosophy of immunology).  A fundamental issue that arises in many of these contexts as well as topics raised above is whether and how one should conceptualize cells as individuals or agents (see the entry on  the biological notion of individual).