Obstructive sleep apnoea (OSA) affects a substantial portion of the population in Finland and worldwide, with prevalence rising. Untreated OSA carries wide-ranging consequences, including cardiovascular, metabolic, cognitive, and mental health issues. The palatine tonsils, located in the oropharynx, consist of lymphatic tissue and contribute to immune defence. In children, sleep-disordered breathing is often caused by enlarged tonsils, and tonsillectomy resolves symptoms. Recently, the procedure has been replaced by the less invasive tonsillotomy, which offers comparable efficacy in paediatric sleep-disordered breathing. Tonsil hypertrophy may persist into adulthood, and traditional tonsillectomy remains a relevant, though understudied, treatment option for adults with OSA. The aim of this thesis was to study the prevalence of adult tonsil hypertrophy, assess complications related to tonsil surgery, and evaluate the long term effectiveness of tonsillectomy in treating OSA. A further objective was to examine whether adults, as shown in children, could benefit from partial tonsil surgery. We analysed tonsil surgeries in our hospital district from 2004 to 2018, identifying adults operated on for hypertrophic indications and assessing long term outcomes in those treated for sleep apnoea. Severe complications were examined using Patient Insurance Centre data. Finally, we conducted a prospective multicentre study to evaluate the efficacy of intracapsular tonsillectomy (ICTE) in adults with large tonsils and OSA. We found that adult tonsil hypertrophy is rare and appears to become rarer with age. Serious complications from tonsil surgery were uncommon and related mainly to routine operations. Although our results with tonsillectomy in OSA are in line with earlier studies, long-term outcomes remain uncertain. Finally, ICTE emerged as a promising alternative to tonsillectomy in adults with OSA. This thesis suggests that many adults with enlarged tonsils, who might benefit from surgery, may remain unidentified within current care pathways. It also highlights the need for further research on long term outcomes of tonsillectomy. In addition, the findings indicate that ICTE may offer an effective alternative to tonsillectomy in adult OSA.