This paper aims to address the relevance of the natural sciences for transcendental phenomenology, that is, the issue of naturalism. The first section distinguishes three varieties of naturalism and corresponding forms of naturalization: an ontological one, a methodological one (with strong and weak variants), and an epistemological one (also with strong and weak variants). In light of these distinctions, in the second section, I examine the main projects aiming to "naturalize phenomenology": neurophenomenology, front-loaded phenomenology, and formalized approaches to phenomenology. The third section then considers the commitments of Husserl's transcendental phenomenology with respect to the three varieties of naturalism previously discussed. I argue that Husserl rejected strong and weak forms of epistemological naturalism, strong methodological naturalism, and ontological naturalism. The fourth section presents the argument that Husserl endorsed a weak, conditional form of methodological naturalism. This point is illustrated with Husserl's proposal of "somatology," a natural science apt to study the corporeality of the lived body. The final section addresses the complementarity and respective limits of the transcendental phenomenological and the natural scientific frameworks. I argue that, on Husserl's account, the function of transcendental phenomenology with respect to the natural sciences is to provide them with an epistemological foundation and an ontological clarification. I suggest that certain natural sciences can be understood, within the transcendental phenomenological framework, as "sciences of constitution," that is, as sciences investigating the contribution of real structures acting as conditions of possibility for the occurrence of certain kinds of comprehensive unities in lived experience.