The study investigated the annual life cycles of four copepod species, thereof two congener pairs, which differentiate in their geographical and/or seasonal distribution patterns: (1) Temora longicornis and T. stylifera, (2) Centropages hamatus and C. typicus. T. longicornis occurs farthermost north and has hardly any overlap with its warm-temperate congener T. stylifera. The two Centropages species represent a homologous pair, with the cold-temperate C. hamatus and the southern-intermediate C. typicus. The distribution patterns of these species suggest that temperature is a major factor for copepod population dynamics and distribution. A field study focused mainly on the reproductive strategies of these species, which was related to temperature and phytoplankton concentration at two stations in the North Sea and the Mediterranean, respectively. In the laboratory, demographic parameters were studied in a temperature range from 2 to 35°C.Temperature plays a central role in the life cycles of small calanoid copepods. In the North Sea, T. longicornis was always present, while C. hamatus was absent from the water column in winter. Both species are known to overwinter as resting eggs in the sediment. The presence of C. typicus was irregular there at its northern distribution margin, underlying high interannual variability. The timing of maximum reproductive activity was similar between species and was recorded in spring, except in C. typicus. Significant correlations were obtained between clutch size and prosome length, which in turn was inversely correlated with temperature. Consequently, temperature controlled egg production via its effect on body size at Helgoland Roads, while food probably was not limiting. In contrast, oligotrophy governed copepod life in the Mediterranean, where reproductive rates were considerably lower, but continuous throughout the year in both T. stylifera and C. typicus. However, no statistical relationship between reproduction and food availability or temperature was observed.The laboratory experiments strengthened the assumption that temperature is one important source of interannual and regional variability. The tolerance limits and optima of survival, reproduction and development distinguished the northern species T. longicornis and C. hamatus from the southern T. stylifera, while C. typicus, which is found in both regions, was intermediate. Thus, thermal preferences can in part explain distribution patterns of copepds. C. typicus was able to tolerate different temperature conditions, resulting in its wide distribution range from the subarctic to the tropics.