Engagement of leukocyte surface receptors by their cognate ligands initiates different cascades of intracellular signaling events. Some of the early events include the activation of protein tyrosine kinases, Jak kinases, serine/threonine kinases, and heterotrimeric GTP-binding proteins. These early events result in the generation of second messengers and the recruitment and modification of adaptor proteins with the resultant formation of multi-subunit signaling complexes. Subsequently, other enzymes such as guanine nucleotide exchange factors (GEFs), small GTPases, phospholipases, phosphatases, lipid kinases and mitogen-activated protein (MAP) kinases are activated, leading to the activation of multiple transcription factors. All these events result in the induction of specific genetic programs that guide the differentiation, activation, and effector activities of leukocytes. A dysregulated immune response can lead to harmful results such as the development of autoimmune disease, allergy or immune deficiency. All of these abnormalities could occur due to some impairment in leukocyte development, defects in anergy, or in apoptotic or other signal transduction pathways. The 4th Leukocyte Signal Transduction Workshop will address specific basic aspects of signal transduction with a major focus on clinical implications of signaling pathways. To this end, the Organizing Committee has brought together a distinguished group of speakers, internationally known leaders in their respective fields, to present and discuss their recent research findings. Similar to its predecessors (Santorini 2000, 2002 and Crete 2005), this workshop is organized to maximize scientific interactions in a most conducive environment.