In its century-long history, allergen immunotherapy (AIT), has shown continuous evolution in terms of the materials and the treatment schedules used, the adequate duration, and the mechanisms of action underlying its clinical efficacy. The passage from the empirical phase of AIT to the era of evidence-based medicine (EBM) was associated with achievement of the highest levels of evidence. This regarded both forms of AIT currently used, represented by subcutaneous immunotherapy (SCIT) and sublingual immunotherapy (SLIT). In particular, SLIT tablet preparations of pharmaceutical quality provided physicians and patients a treatment whose efficacy had been confirmed with the highest level of scientific evidence and improved the credibility of AIT for the entire medical field. However, further advances are needed for AIT in terms of optimal patient selection and the required dosage, as well as the quality and composition of the allergen extracts, factors favouring compliance, and the most appropriate duration capable of maintaining the clinical benefit over time.