All living cells need to perceive different signals from their surroundings. Each cell possesses a complex molecular machinery composed of receptors and other regulatory and accessory proteins that participate in transmission and routing extracellular signals into the cell interior. G-protein-coupled receptors (GPCRs) represent the largest group of membrane-bound receptors, which are extensively studied because of their involvement in the regulation of many cellular functions and processes. The first part of this book provides a comprehensive review of current knowledge regarding GPCRs, G-proteins and their signalling pathways. In the next section, the author describes her own experiments dealing with the thyrotropin-releasing hormone (TRH) receptor. She investigated how sustained stimulation of TRH receptors can affect cellular processes and analysed protein complexes of TRH receptors and their cognate Gq/11 proteins participating in signal transduction. This book can help students and professionals to get an idea about cellular signalling mediated by the receptors coupled with G-proteins, as well as about some experimental approaches for the exploration of these molecules.