Sublingual immunotherapy (SLIT) is a well-established treatment option for allergic rhinitis in several European countries, but it is considered investigational in the United States. Studies conducted in Europe provided a large body of evidence supporting the safety and efficacy of SLIT, but those studies used allergen products that are different from those that are likely to be approved in the United States, and many of them were not controlled, randomized, double-blinded trials. This review summarize research conducted on the efficacy, safety, and mechanisms of SLIT published during the past year, with a focus on ragweed and grass antigens. Results of recent US studies document the safety and efficacy of SLIT and have started to yield insight into the mechanisms of SLIT.