Cells utilize an array of mechanisms to sense and respond to their environment. Typically, the initiation of a cellular response begins with the interaction(s) between cellular receptors and their cognate ligands. These events trigger a cascade of sequential, intracellular events within the cell. Specific assays and reagents are important in order to characterize these protein-protein interactions and modifications, as well as to measure changes in cell surface proteins or those released from the cell. Together, cell signaling systems allow cells to detect, respond to and communicate with other cells, both locally as well as systemically.