Toll-like receptors (TLRs) recognize specific molecular patterns of pathogenic microorganisms, including bacteria, fungi, protozoa, and virus. Stimulation of TLRs triggers gene expression involved in innate immune response and further instructs development of antigen-specific adaptive immunity. Molecular mechanisms by which TLRs activate innate immunity are now being elucidated through analysis of TLR-mediated signaling pathways. TLR signaling originates from the cytoplasmic Toll / IL-1 receptor (TIR) domain, which is conserved among all TLRs. In addition, recent evidence indicates that TIR domain-containing adaptors, such as MyD88, TRIF, TIRAP, and TRAM, play essential roles in TLR signaling. MyD88 is essential for inflammatory cytokine production via all TLRs, whereas TRIF mediates a MyD88-independent induction of type I IFNs via TLR3 and TLR4. TIRAP is specifically involved in TLR2-, and TLR4- mediated MyD88-dependent pathway, and TRAM acts in the TLR4-mediated TRIF-dependent pathway. Therefore, the specific functions of individual TLRs can be elicited by utilizing different combinations of TIR domain-containing adaptors. These recent progresses have made us aware of the fact that innate immunity possesses a skillful system to detect microbial invasion in the host and trigger appropriate immune responses.