In this review we will focus on the application of polymeric nanoreactors in green catalysis by highlighting their structure and ability to encapsulate and shield catalysts. Four different types of nanoreactors will be described, namely micelles, polymersomes, dendrimers and nanogels. The choice of discussing these nanoreactors stems from their accredited relevance in the field of catalysis and the significant number of examples published in literature. The advantageous aspects of these four classes of nanoreactors over non-supported homogeneous systems include: 1) the site isolation of reactive components (enabling cascade reactions), 2) the ability to convert hydrophobic substrates in water (under green conditions), and 3) the facile catalyst recovery. All these attractive features are covered in this review. Moreover, in this review we have not attempted to be comprehensive, but we rather want to illustrate the application potential of these nanoreactors with some illustrative examples of the most relevant classes of organic reactions (performed in water), which should interest both academia and industry.