This repository contains the BrainTeaser Ontology (BTO) whose purpose is to jointly model both Amyotrophic Lateral Sclerosis (ALS) and Multiple Sclerosis (MS). BTO serves multiple purposes: to provide a unified and comprehensive conceptual view about ALS and MS, which are typically dealt with separately, allowing us to coherently integrate the data coming from the different medical partners in the project; to seamlessly represent both retrospective data and prospective data, produced during the lifetime of BRAINTEASER to allow for sharing and re-using the BRAINTEASER datasets according to Open Science and FAIR principles. BTO is innovative since it relies on very few seed concepts - Patient, Clinical Trial, Disease, Event - which allow us to jointly model ALS and MS and to grasp the time dimension entailed by the progression of such diseases. Indeed, the core idea is that a Patient participates in a Clinical Trial, suffers from some Diseases, and undergoes Events. These Events are different in nature and cover a wide range of cases, e.g. Onset, Pregnancy, Symptom, Trauma, Diagnostic Procedure (like evoked potentials or ALS-FRS questionnaires) Therapeutic Procedure (like Mechanical Ventilation for ALS or Disease-Modifying Therapy for MS), Relapse, and more. Overall, this event-based approach allows us to model ALS and MS in an unified way, sharing concepts among these two diseases, and to track what happens during their progression. The full BTO documentation can be accessed at https://brainteaser.dei.unipd.it/ontology/. The BTO has been used to share the BRAINTEASER ALS and MS Datasets.