 All very well, you might say: biologists certainly should think microbes are important. But what about philosophers? What can microbes tell them? There are numerous debates in philosophy of biology in which neglecting microbes would seriously weaken, or at least bias, any philosophical conclusions that might be reached. This entry will focus on three connected philosophy of biology topics: views about species, the tree of life, and major transitions in evolution. A discussion of a broader philosophy of science topic, philosophy of modelling, follows in  Section 3.