The latest evidence on the mechanisms, efficacy, and safety of sublingual immunotherapy (SLIT) was reviewed. Interleukin (IL) 35 and IL-35-producing regulatory T cells were assessed as new biomarkers for SLIT responsiveness. A detailed analysis of clinical studies, including timothy grass pollen, 5-grass pollen, ragweed, and house-dust mite SLIT tablets, was provided, including a comparative analysis of efficacy and safety of SLIT versus subcutaneous immunotherapy.