The Special Issue on ‘Towards Circular Cities – Nature-based solutions for creating a resourceful circular city’ comprises six contributions prepared by the COST Action Circular City (https://circular-city.eu/). The Action aims to establish a network of researchers and stakeholders testing the hypothesis that: ‘A circular flow system that implements nature-based solutions (NBS) for managing nutrients and resources within the urban biosphere will lead to a resilient, sustainable and healthy urban environment.’

The six papers present the first outcome of the COST Action, triggered at the workshop, held on 13–15 February 2019 at BOKU University Vienna, Austria. They are the starting point towards deeper exploration of how NBS can contribute to circular cities, gathering state-of-the-art knowledge of one of the biggest COST Actions so far. In the first paper, we introduce the COST Action Circular City by describing its main objectives and aims as well as the general activities of the Action. The remaining five papers review the start-of-the-art of the Action's five Working Groups, i.e. Built environment, Sustainable urban water utilisation, Resource recovery, Urban Farming, and Transformation tools.

More than 100 persons from 35 COST countries contributed to the review papers. Preparing the review papers was an interdisciplinary effort in the true spirit of a COST Action network. It aimed at including the vast amount of existing knowledge in the network by first adopting a common definition of NBS and establishing a common language and understanding among disciplines. We highly appreciate the willingness and the effort of authors to contribute to this interdisciplinary work.

We do hope that the Special Issue ‘Towards Circular Cities’ will create interest for activities of the COST Action Circular City but also will help the new Blue-Green Systems journal to reach new readers.