Visual impairment (VI) due to eye diseases remains a significant healthproblem worldwide and, also, in Europe. There are an estimated 15 million peoplesuffering from moderate or severe visual impairment in Western Europe. VI has asignificant impact on the quality of life by reducing functional status and interferingwith the ability of the subject to maintain independence in a safe manner. Prevalenceof VI needs to be estimated regularly so that the progress of the vision health of apopulation can be evaluated and monitored. In addition, it is important to ascertainthe causes behind VI so that health programs can be designed to lower itsprevalence. There is currently a lack of epidemiological information about theprevalence and causes of VI in Portugal. Therefore, the aim of this thesis was todetermine the prevalence of VI in a large region Portugal using data from lists ofcases of VI.Capture-recapture models have been applied in several disciplines, asbiomedical sciences, epidemiology or ecology, to estimate the size of populations. Inparticular, they have been used to estimate the prevalence of several diseases orconditions. Developing these inferential models is of great importance to avoid thehigh costs and unreasonable time spending of cross-sectional studies. However,applying capture-recapture models is challenging, as they are very sensitive to listdependence and possible capture rates heterogeneity among subgroups of thepopulation. In particular, applying these models to human population samples isadditionally challenging, as in most epidemiologic studies only a small number oflists are available. There are two main differences between human and wildlifepopulations. First, human population lists generally have not a well-defined timeorder. Second, in wildlife studies there are often more trapping samples than inhuman population studies. In most epidemiologic surveys, only two to four lists areavailable. This can be problematic and is an additional difficulty when applyingcapture-recapture models in the context of human populations.The main objective of this work was to estimate the prevalence of VI usingcapture-recapture models. We estimated a crude prevalence of1.97%,95%CI=[1.56,2.54] to the Northwest of Portugal in the time period between2014 and 2015, specifically at the regions of Minho and Douro Litoral. Almost 2 ofevery 100 inhabitants of the Portuguese Northwest suffer from visual impairment.This prevalence value is in line with the values in some countries, particularly withSpain. Diabetic Retinopathy was the main cause (31%), followed by Cataract (15%),Age-related Macular Degeneration (14%) and Glaucoma (10%). This thesis provides asignificant contribution to the understanding of the CR methodology in human populations and for the knowledge of the epidemiological information about VI inPortugal.