This thesis deals with various aspects of twin anemia polycythemia sequence (TAPS). TAPS is a condition that can develop due to unbalanced feto-fetal blood transfusion through minuscule vascular placental anastomoses in monochorionic twin pregnancies, causing the donor twin to become anemic and the recipient twin to become polycythemic. In this thesis we show that a difference in middle cerebral artery peak systolic velocity (MCA-PSV) > 0.5 Multiples of the Median (MoM) has a high diagnostic accuracy for the the antenatal diagnosis of TAPS. For postnatal diagnosis of the condition, inspection of the color of the maternal side of the placenta can be of great value. Furthermore, we present the results of a large international registry, and report on outcomes after different treatment options for TAPS. As the best treatment for TAPS is unclear, we propose the protocol of The TAPS Trial (a multicenter open-label international RCT) to investigate the potential beneficial effect of fetoscopic laser surgery for the outcome in TAPS twins . In the last chapters of this thesis we discuss short- and long-term outcome and show that TAPS donors show significantly higher rates of perinatal mortality and long-term neurodevelopmental impairment than their recipient co-twins. Afdeling Kindergeneeskunde, Afdeling Verloskunde LUMC, LUMC, Mieke Kerfhof, Nederlandse Vereniging voor Ouders van Meerlingen, Stichting Oranjekliniek, Stichting IDB.LUMC / Geneeskund