The past two decades have provided a vast array of information about growth hormone (GH) at the molecular, cellular, and organismic level. GH is an anterior pituitary, peptide hormone that binds to cell surface receptor via a two-step mechanism. The resulting change in receptor structure increases the affinity and activation of JAK-2, which phosphorylates itself and the receptor, forming high affinity binding sites for signaling proteins. The recruitment of numerous signaling molecules leads to the activation of multiple pathways that control gene transcription and metabolism. GH elicits its effects on a wide variety of tissues, including cell of the immune system that also produce small amounts of GH. Different cell types of the immune system respond to GH in a variety of functional assays. Models of GH defficiency are not usually associated with clinically relevant immonudeficiency that may in part be explained by cytokine redundancies. The theraphuetic use of GH, however, appears to be for the most part beneficial and needs to be studied further.