The development of immune responses is significantly influenced by emerging patterns of cytokine expression in activated CD4+ T cells. Recent efforts have clarified both cellular and molecular mechanisms, and within the past year include significant observations on potential sources of IL-4 leading to Th2 development against certain pathogens, and insights into early responses and genetic susceptibility to experimental murine Leishmaniasis and transcriptional regulation of the IL-4 locus. Advances in Th1 development have included greater understanding of IL-12 receptors in Th1 development, data regarding IFN-gamma gene expression and clarification of the action of the new cytokine IL-18.