The article summarises the state of research on Poland’s population in the late Middle Ages and at the beginning of the Early Modern Times; at the same time it indicates the necessity and possibility of correction of the present assessments. So far historical demographers have been concentrating on counting the country’s population at two temporal points: 1340 and 1580. In the first case they concentrated on the registers of Saint Peter’s pence, and in the other – on the tax registers. The two sources do not inform directly on the size of the population, but rather on the amount of paid taxes: per head, per household, the size of acreage or the value of property. For almost a hundred years a special methodology has been created to use those sources for the needs of demography, and in the literature there have appeared several conversion factors to assess the size of the population. In spite of all that and because of the imperfection of the source basis, a defective edition and different conversion factors applied by various researchers, the assessments of the size and structure of the population of the Kingdom of Poland in the late Middle Ages and at the beginning of the Early Modern Times that have been made so far, are not entirely satisfactory.