TLR9 (Toll-like Receptor 9) is a member of the TLR (Toll-like receptor) family that plays a fundamental role in pathogen recognition and activation of innate immunity. TLR9 is expressed by numerous cells of the immune system such as B lymphocytes, monocytes, natural killer (NK) cells and plasmacytoid dendritic cells. It is expressed intracellularly, within the endosomal compartments and functions to alert the immune system of viral and bacterial infections by binding to DNA rich in CpG motifs. This nucleotide-sensing TLR is activated by unmethylated cytidine-phosphate-guanosine (CpG) dinucleotides (also called oligodinucleotides, ODNs), and consequently signals via MYD88 and TRAF6, leading to NF-κB activation, cytokine secretion and the inflammatory response and eventually activation of a Th1-biased adaptive immune attack. Specific TLR9 agonists have been shown to have substantial antitumor activity, making the TLR9 signaling pathway an interesting target for therapeutic research.