Selective expansion and activation of a very small number of antigen-specific CD4(+) T cells is a remarkable and essential property of the adaptive immune response. Antigen-specific T cells were until recently identified only indirectly by functional assays, such as antigen-induced cytokine secretion and proliferation. The advent of MHC Class II tetramers has added a pivotal tool to our research armamentarium, allowing the definition of allo- and autoimmune responses in deeper detail. Rare antigen-specific CD4(+) cells can now be selectively identified, isolated and characterized. The same tetramer reagents also provide a new mean of stimulating T cells, more closely reproducing the MHC-peptide/TCR interaction. This property allows the use of tetramers to direct T cells toward the more desirable outcome, that is, activation (in malignancies and infectious diseases) or Th2/T regulatory cell deviation, anergy and deletion (in autoimmune diseases). These experimental approaches hold promise for diagnostic, prognostic and therapeutic applications.