Binding of epidermal growth factor (EGF) to the EGF receptor (EGFR) initiates signal transduction, ultimately leading to altered gene expression. Ligand-activated EGFR is also rapidly internalized and then targeted to lysosomes for degradation or recycled back to the plasma membrane. Endocytosis is a major regulator of EGFR signaling. Therefore, elucidation of the mechanisms of EGFR endocytosis is essential for a better understanding of EGFR biology. In order to achieve a comprehensive analysis of these mechanisms, reliable methods for measuring the rates of EGFR protein turnover and the rate parameters for individual steps of EGFR endocytic trafficking must be employed. The protocols in this unit describe methodologies to measure the rates of EGFR synthesis and degradation, to monitor EGF-induced down-regulation of surface EGFR, to measure the kinetic rate parameters of internalization, recycling, and degradation of radiolabeled EGF, and to perform radioiodination of EGF by the chloramine T method.