 Like the concept of an organism that it putatively supplements or supplants, that of the holobiont encompasses a huge diversity of entities. These include macro-organisms and the microbial endosymbionts living within their cells (such as Chlamydia and other obligate parasites); those that live beyond their cells but in close symbiotic relations (such as cyanobacteria); and the multispecies microbiota that inhabit the human intestine (Booth 2014). There are challenges in how to delineate individual holobionts that, as we will see in the next section, may be met by embracing particular physiological criteria for individuation (Pradeu 2012). Yet those challenges have suggested to some that the take-away lesson from reflection on holobionts is that emphasis should be placed not on another kind of individual, a holobiont, but instead on the process of holobiosis (Doolittle &amp; Booth 2017). It remains too early to tell whether the concept of the holobiont will live up to its still-early promise of revolutionizing thinking about biological individuals; this will likely be an area of emerging and exciting work for the next several years.