The copepod Calanus finmarchicus has an ecological key position in the northern Atlantic pelagic food web and its life is characterized by diel and seasonal rhythmicity. Neither diel nor seasonal rhythmicity of C. finmarchicus are understood with regard to their mechanistic regulation. Endogenous clock systems are central in controlling rhythms in various terrestrial species, but have hardly been investigated in marine organisms. This thesis shows that C. finmarchicus possesses an endogenous circadian clock, that regulated 24h rhythms of gene expression, metabolic activity and vertical migration behavior. The thesis further suggests that clock-based day length measurement and an endogenous annual clock is involved in the regulation of seasonal rhythmicity. The findings on C. finmarchicus’ timing systems are further related to the extreme light conditions in polar environments, discussing potential effects of climate chance on the copepods rhythmicity and biology.