Sublingual immunotherapy is gaining widespread attention as a viable alternative to subcutaneous immunotherapy for the treatment of allergic rhinoconjunctivitis. In addition, sublingual immunotherapy has been studied in other allergic disorders including asthma. However, a review of published studies indicates that there are deficiencies and considerable heterogeneity in both design and data interpretation of sublingual immunotherapy studies. These deficiencies have made it somewhat difficult to assess the appropriate place of sublingual immunotherapy in guidelines for the therapy of allergic diseases. Moreover, several unpublished oral and sublingual immunotherapy studies in the United States failed to meet primary endpoints. This article reviews data from sublingual immunotherapy trials and makes recommendations about appropriate designs of future sublingual immunotherapy studies. It is hoped that these recommendations will result in more adequately designed sublingual immunotherapy trials to facilitate the appropriate placement of this therapy to treat patients with allergic rhinoconjunctivitis and other allergic diseases.