Myeloproliferative neoplasms (MPN) are a group of clonal haematopoietic stem cell diseases usually associated with increased production of blood cells (red blood cells, white blood cells, and platelets), splenomegaly, and other symptoms (fatigue, pruritus, night seat, early satiety, abdominal discomfort, weight loss, and recurrent fever). In addition, the main complications of MPN involve thrombotic (venous and arterial) events and less frequent progression to acute leukaemia and death. Usually, MPN are diagnosed during the 6th decade, but it is now wellrecognised that a small proportion of patients (<20%) are diagnosed below the age of 40 years. The diagnosis of MPN in young patients, in addition to the extension of life expectancy, has revealed emergent questions regarding thrombotic risk, progression, disease evolution, and pregnancies. Data on young MPN are limited, and this original thesis presents three papers illustrating two aspects of this population. The first two papers focus on the description of this population regarding characteristics of diagnosis, management, and prognosis. Moreover, the first paper presents the largest real-world study of young patients diagnosed with MPN and provides reliable information on this population. Finally, the third paper focused on the management of pregnancy in this population. MPN are associated with a high risk of foetal and maternal complications. The use of interferon is part of the treatment to reduce complications during pregnancy. This paper presents the first and largest series of patients treated with peginterferon alfa-2a during pregnancy, a more recently developed form of interferon with better tolerance than the previously used interferon alfa-2a. This selected work is put into perspective with the current knowledge of this rare population of young patients with MPN and its management.