Assessment of clinical outcomes is essential to evaluate the efficacy of allergen immunotherapy, both in clinical trials and in daily clinical practice. "Primary outcomes" used in sublingual and subcutaneous immunotherapy trials include the measurement of symptoms, the use of concomitant medications, and/or a combination of both of these measures. "Secondary outcomes" that have been measured include individual symptom scores, provocation allergen tests, visual analog scales, and health-related quality of life assessments. Currently, there is a lack of consistency in the selection of outcome parameters used, which represents a major problem when comparing the same therapeutic intervention. In this review, we examined the different primary and secondary outcome parameters that have been used in clinical trials; reviewed recommendations by European and US guidelines, and allergy groups; and explored which outcomes may be applicable for use in clinical practice.