Bacterial CpG-DNA sequences not only serve for genetic information but act as "danger" signal to alert immune cells such as macrophages and dendritic cells. CpG-DNA directly causes activation of these members of the innate immune system yielding in up-regulation of co-stimulatory molecules and pro-inflammatory cytokines. As such they act as "natural" adjuvants to promote protective Th1 polarized T cell immune responses. This book is the first comprehensive review of the data emerging in this field.