I examine the degree of specialization in various sub-fields of philosophy, drawing on data from the PhilPapers Survey. The following three sub-fields are highly specialized: Ancient philosophy, seventeenth/eighteenth century philosophy, and philosophy of physics. The following sub-fields have a low level of specialization: metaphilosophy, philosophy of religion, philosophy of probability, philosophy of the social sciences, decision theory, and philosophy of race and gender. Highly specialized sub-fields tend to require extensive knowledge in some area beyond the typical training of a philosopher, (...) and outside of philosophy proper. In addition, there is a correlation between sub-field size and degree of specialization. Larger sub-fields tend to be more specialized. (shrink)