In 1995, Signal Transduction Protocols, edited by David A. Kendall and Stephen J. Hill, was published in the Methods in Molecular Biology series. This second edition represents an update to that previous work with an emp- sis on new methodologies that have developed in the last few years. The goal, then and now, is to provide procedures written by experts with first-hand ex- rience in a detail that goes far beyond what is generally encountered in the "methods" section of most journals and thus actually permits a particular p- cedure to be replicated. In addition, we have had as a secondary goal the id- tification of protocols for the assay of general classes of signal transduction components that, ideally, can be adapted to the assay of any member of that class. The ability to do this has resulted in large part from the use of affini- based assays, the ease with which specific proteins can be specifically tagged, and an explosion in the availability of highly specific antibodies from comm- cial sources, especially antibodies raised against signaling proteins of human origin. The number of available approaches is, fortunately for those working in signaling research, far too great to fit within the confines of this volume, so hard choices as to what to include had to be made.