The 2016 ExoMars Trace Gas Orbiter (TGO) is the first in a series of Mars missions to be undertaken jointly by two space agencies, ESA and Roscosmos. A key goal of this mission is to gain a better understanding of methane and other atmospheric gases that are present in small concentrations (less than 1% of the atmosphere) but nevertheless could be evidence for possible biological or geological activity. One of the instruments of TGO is CaSSIS – Colour and Stereo Surface Imaging System. A high resolution camera (5 metres per pixel) is capable of obtaining colour and stereo images over a wide swath. CaSSIS will provide the geological and dynamical context for sources or sinks (sinkholes) of trace gases detected by two other instruments: NOMAD and ACS. Poland (CBK PAN) is responsible for delivery of CaSSIS Power Supply Unit.