1. Introduction 321

2. Tyrosine kinase receptors 322

3. Histidine kinase sensors 325

4. G-protein coupled receptors 327

5. Principles 328

6. Acknowledgments 329

7. References 330

Biological cells need to be responsive to various stimuli, primarily chemical ligands from their environments. Specific receptor molecules embedded in the plasma membrane detect the different biochemical signals that impact the cell, and these receptors are the conduits for transmission of this information to the cell interior for action. There are several classes of signal transduction receptors and many specific receptors within each of the major classes. This review emphasizes the structural biology of three major classes of transmembrane receptors – tyrosine kinase receptors, histidine kinase sensors, and G-protein coupled receptors. Biophysical principles that govern the processes of signal transduction across cell membranes are also discussed.