Due to its Arctic location at 79°N, Kongsfjorden in Svalbard experiences strong seasonality in light climate, changing from polar night to midnight sun. Sea ice conditions and the optical properties of seawater further modify the amount and the spectral composition of solar radiation penetrating into the water column, thus defining the underwater light climate in Kongsfjorden. Light represents one of the major shaping factors for the entire marine ecosystem. A number of studies focusing on implications of the underwater light for marine organisms have beenconducted in Kongsfjorden, generating diverse datasets on seawater optical properties, scattered over time and space. This review synthesizes the fragmentary information available from the literature as well as presenting some unpublished data, and discusses the underwater light climate and its main controlling factors in Kongsfjorden. Furthermore, we provide a short synopsis about the relevance of light for different components of an Arctic marine ecosystem, exemplified by studies carried out in Kongsfjorden. Due to its year-round accessibility and its high-Arctic location, Kongsfjorden has become a prime fjord for studying how the strong seasonal changes in light availability, ranging from polar night to midnight sun, affect marine life with respect to primary production, behavioural aspects and synchronization of growth and reproduction.