District heating (DH) can become a key infrastructure for achieving climate targets in the heating sector. In order to support the uptake of renewables in the DH sector, the European Commission proposed to open DH infrastructures to third parties. This will allow independent heat producers to supply heat produced from renewable energy sources and from waste heat to consumers connected to the grid. This paper develops a better understanding of the complexity associated with the introduction of third party grid access (TPA). We will analyse the heterogeneous institutional set-up of DH markets in the EU and discuss competition and market boundaries in the heat market. Based on this, the paper investigates the technical, regulatory and economic challenges that arise from the practical implementation of TPA. We conclude that TPA alone will not be sufficient to support the expansion of renewables in the DH sector. Complementary policy measures will be necessary to transform the DH sector towards 4th generation DH systems that will become an integrated element of a smart energy system.