In recent years, researchers have increasingly focused on the modulation of regulatory T cell (Treg) function to interfere with the outcome of virtually every type of immune response. For a long time, specific in vivo targeting of Tregs was precluded due to the lack of appropriate markers. Only after the discovery of Foxp3 as a Treg-specific transcription factor, was the development of Treg-specific mouse models feasible. We generated DEREG mice (DEpletion of REGulatory T cells), a BAC (bacterial artificial chromosome) transgenic mouse line, which allows direct in vivo analysis and depletion of this exceedingly important cell type. Our DEREG mice carry a DTR-eGFP transgene under the control of an additional Foxp3 promoter, thereby allowing specific depletion of Treg by application of diphtheria toxin at any desired point of time during an ongoing immune response. This chapter will elaborate the advantages and disadvantages of employing different genetic approaches and discuss further parameters used in the studies focusing on employment of diphtheria toxin and its degree of general toxicity in mice. Additionally, we will address the question: to which extent DEREG mice are suitable for studying the effect of long-term Treg depletion during specific immune responses.