Vaccines aim at developing protective acquired immunity against pathogens. Optimal activation of acquired immunity involves activation of innate immune system, which is often achieved by chemical substances called adjuvant. Recently, molecules recognizing pathogen-associated molecular patterns called pattern recognition receptors(PRRs) have been identified and their signaling mechanism has been rapidly elucidated. Our increased understanding on molecular mechanism of innate immune recognition starts to clarify how adjuvant activates innate immune system and consequently achieves efficient acquired immunity via PRRs. In this review, we will discuss recent findings on the roles of PRRs as adjuvant receptors.