In order to protect the components of natural environment, each landfill must be properly secured and the monitoring program should be adopted. This study aims to present a comparative analysis of groundwater quality at selected landfill sites in Poland and the Czech Republic, with a special attention given to the levels and temporal changes of heavy metals (HMs) concentrations measured in collected groundwater samples. A secondary objective was to detect possible leakages of pollutants from the landfill body, into the groundwater, and further into the environment. The assessment of groundwater quality was based on a comparison of HMs concentrations with standards provided by the European environmental laws. On the basis of the long-term monitoring period, it was revealed, for the Polish landfill site, that the groundwater quality is improving over time, especially due to remedial works applied. For the Czech landfill, it was observed that the quality of groundwater is not negatively affected by the operation of the landfill, but in the immediate vicinity of the landfill, the groundwater quality is significantly affected by the agricultural use of neighbouring lands, as well as by the storage of construction and demolition wastes. The results showed that the leachate did not leak outside the landfills, especially due to minimal concentrations of HMs, measured in groundwater samples, taken from the piezometers located in the outflow direction from the landfills.