This is a phase I, single-center, randomized, placebo-controlled study of sublingual immunotherapy (SLIT) in pediatric and adult subjects with both house dust mite (HDM) and timothy grass (TG) allergies. We will evaluate whether Dermatophagoides farinae (DF) and/or TG allergen SLIT is safe in children and adults. We will also determine whether treatment with DF and/or TG SLIT reduces the severity of allergic symptoms (allergic rhinitis, allergic conjunctivitis) and enhances their resolution. The study will also evaluate whether SLIT provides a robust durability of response once it is terminated. The dosing-phase of the study will last up to 12 months. In addition, a follow up period of 2 years will occur. Approximately 10 subjects will be on placebo, and 20 on active treatment.