In nature, circular polarization detection is unique to bio-systems, since human eyes cannot detect polarization information. Various animal species successfully use it for defense, signaling and orientation. It may be of great interest to reproduce these functionalities in artificial photonic devices, being able to distinguish different handedness of circular polarization, and to possibly integrate this operation with other photonic systems. However, currently available artificial circular polarizers are inherently narrow bandwidth, bulky and with poor efficiency. Here, we put forward some exciting ideas to employ individual and stacked plasmonic metasurfaces to generate or to filter circular polarizations. We model a single ultrathin metasurface as a shunt circuit element in terms of its admittance tensor and show that under certain conditions one can exploit the exotic features of plasmonic materials to realize ultrathin circular polarization filters with broad bandwidth of operation.