Abstract
mTOR is a key regulator of cell growth and size, and its activity is often dysregulated in a wide variety of diseases. The mTOR signaling pathway is also a therapeutic target for many diseases, including cancer. Immunohistochemistry is a powerful method to assess mTOR activity in clinical/histological samples, however, care should be taken in choosing the targets for determining mTOR activity due to the complexity of its regulation. This chapter describes the most up-to-date methods for visualizing mTOR activity by immunohistochemistry using commercially available antibodies, including considerations for validating new antibodies for assessing mTOR signaling.