New trends in experimental marine research require long term monitoring of the seas and oceans with higher data resolutions than those which can be obtained from oceanographic vessels, buoys or autonomous se nsors. Subsea observation platforms are the response to these needs. These infras tructures are complex systems, precisely designed to ensure they are flexible, robust and stable and also capable of managing a wide range of sensors and instruments in r eal time with minimum maintenance. Some of the most commonly used instruments fo r ocean research are based on optical and image sensors, and in this chapter we provi de a review of the key components of a subsea observatory and the main challenges facing optical and image sensors used in multidisciplinary and multiparameter underwater observatories.