Landfill leachate is produced many years after a landfill site closes. Hence, treatment by “natural methods”, as e.g. constructed wetlands, with low management requirements is attractive. Constructed wetlands usually provide both shallow and deep areas with aerobic and anaerobic zones, which is suitable for nitrification followed by denitrification of the ammonium-rich leachates. Full-scale treatment systems are influenced by climatic variables that affect the microbial community. Also, the operational strategy can have a considerable impact on both activity and composition of the microorganisms. Many studies have measured inflow/outflow water quality in treatment systems. Such “black box” studies describe the treatment efficiency but add little to an increased understanding of theorganisms performing the treatment or the spatial distribution of their activities and treatment processes. In this thesis we investigated seasonal and annual changes in potential nitrification and denitrification, and in the corresponding bacterial communities in constructed wetlands treating landfill leachates. Variations in the potential activity in full-scale systems were investigated over several years, using short-term incubation. The composition of the bacterial communities was investigated using a group-specific PCR primer pair targeting the 16S rRNA genes or a primer pair targeting the funtional gene nosZ. The PCR products were analysed by denaturing gradient gelelectrophoresis and subsequent nucleotide sequencing and phylogentic analysis. A stable ammonia-oxidising bacterial (AOB) community composition and potential ammonia-oxidation (PAO) were detected in the system with a year-round operation. On the other hand, changes in the AOB community composition which followed the operational schedule were detected in the overland flow area (OFA) running seasonally. Furthermore, the influence of operational strategy was indicated by a low PAO in the wastewater overland flow area and compact constructed wetland receiving high hydraulic loads, indicating the value of aeration. Higher PAO was detected in the OFAs where the hydraulic load followed literature guidelines. All systems supported diverse AOB communities, represented by several Nitrosomonas and Nitrosospira populations. The number of different populations detected in these wetlands was much higher than reported in municipal wastewater treatment plants, and differed from those in a parallel OFA treating municipal wastewater. Furthermore, the large variation in both potential activity and sequences detected in replicate samples suggests that such systems are spatially heterogenic.