Allergen extracts for sublingual immunotherapy (SLIT) are currently marketed by several manufacturers, with administration schedules and amount of allergen(s) quite variable in the different products, although almost all are standardized biologically or immunologically. The allergen extracts for SLIT are available in two main pharmaceutical forms: solution to be delivered by drop-counters, pre-dosed actuators (mini-pumps) or disposable single-dose vials; tablets with appropriate composition that allows a slow (1-2 minutes) dissolution in the mouth in contact with saliva. In Europe, SLIT is prescribed in general for one or a few allergens, and mixtures are less used, though there is no immunological contraindication to give multiple allergens. SLIT traditionally involves a build-up phase and a maintenance phase with the top dose. The build-up phase has usually the duration of 4 - 6 weeks. The patient must start with the lowest concentration and gradually increase, using the different dosage preparations, until the maintenance dose is reached. Rush and ultra-rush inductions have been introduced, based on the safety profile of SLIT that is very favorable. For these reasons it has been suggested that an updosing phase maybe even not necessary. The no-updosing approach would result in a treatment that is more patient-friendly and convenient to manage. Indeed, the most recent randomized trials were performed with the no-updosing regimen and their results in term of safety were as favorable as the studies performed with the traditional updosing approach. The currently recommended duration of SLIT is comprised between 3 and 4 years depending on the clinical response in single patients.