The main purpose of this thesis was to examine whether the Danish National Patient Registry (NPR) could be used to monitor and assess the quality of cataract surgery in Denmark by studying the risks of two serious postoperative complications following cataract surgery - retinal detachment (RD) and postoperative endophthalmitis (PE). The thesis consists of four retrospective studies. In the first study (paper I), we used data from the NPR in the calendar period 2000-2010 to investigate the risk of pseudophakic retinal detachment (PRD) using the fellow non-operated eyes of the patients as reference. The study showed that over a 10-year study period, the risk of PRD was increased by a factor of 4.2 irrespective of sex and age. The risk of PRD was highest in the first part of the postoperative period and then gradually decreased but remained statistically significantly higher than the risk of RD in non-operated fellow eyes up to 10 years after cataract surgery. The epidemiology of RD in the non-operated fellow eyes was different from the epidemiology of RD in the background population as young men had the highest risk of RD in the non-operated fellow eyes. This means that the absolute risk of PRD was highest for young men because they had a higher risk of RD before they underwent cataract surgery. In the second study (paper II), we used data from the NPR and reviewed patient charts to assess the risk of PE after cataract surgery performed in public eye departments and private hospitals/clinics in the study period 2002-2010. The overall risk of PE among the seven public eye departments was 0.36 per 1000 registered cataract operations, and the PE risk among the departments was homogeneous. The overall risk of PE among the 28 private hospitals/clinics was 0.73 per 1000 registered cataract operations, and the risk among the private hospitals/clinics was heterogeneous. Most private hospitals/clinics had a risk of PE that was lower than or similar to the risk of PE after registered cataract surgery in public eye departments, but six private hospitals/clinics had a statistically significantly higher risk of PE compared to the public eye departments. We used PE as a proxy measure of the registration of cataract surgery and found that 98% of the cataract operations performed in public eye departments were registered in the NPR while only 38% of the cataract operations performed in private hospitals/clinics were registered in the NPR. In general, the coding of the PE cases was not uniform and the lack of registration by the private hospitals/clinics meant that the NPR could not be used to monitor the true risk of PE. NPR data were also used in the third study (paper III) to examine whether patients who had surgical intervention for PE following cataract surgery with either a pars plana vitrectomy (PPV) or a vitreous tap (VT) had a higher risk of subsequent surgical complications. There was no statistically significant difference in the overall risk of complications among the two groups, but the risk of surgery for vitreous opacities was statistically significantly higher for patients who underwent a VT. A surgical complication occurred in 27.3% of the patients and 9.9% of the patients developed more than one surgical complication. Ninety-seven per cent of the primary surgical complications occurred within the first 5 months. The risk of surgical complications in this study was similar to or higher than the risk of complications in the landmark Endophthalmitis Vitrectomy Study from the early 1990s. In the fourth study (paper IV), we used data from all three Danish cataract registries to describe the epidemiology of cataract operations performed in public hospitals and private hospitals/clinics in the study period 2004-2012. Again, PE was used as a proxy measure of the registration of cataract surgery. There were several noticeable differences in the epidemiology of the cataract operations performed in public hospitals and private hospitals/clinics. Patients who had cataract surgery in public hospitals had a statistically significantly higher mortality compared to patients who had cataract surgery in private hospitals/clinics during the entire period. The decrease in the mean age at first eye cataract surgery in private hospitals/clinics was statistically significantly greater compared to public hospitals during the study period. There was a statistically significantly shorter median time interval between first and second eye cataract surgery at private hospitals/clinics compared to public hospitals during the entire study period. The study showed that only 54% of the cataract operations performed in private hospitals/clinics that led to PE were registered. The lack of registration of cataract surgery is the main reason why the NPR has limitations when used as a tool to monitor and assess the quality of cataract surgery in Denmark.