The development of High Altitude Pseudo-Satellites (HAPS) has reached an outstanding level of maturity, with several flight demonstrations that augur soon to have operational capabilities. ESA, aware of the synergies among satellite and HAPS services, set up the HAPPIEST study to analyse the impact of the erruption of aerostatic HAPS into the telecommunications market, currently covered by space and terrestrial networks. Furthermore, some complementary payloads can serve interesting applications such as Earth observation and navigation services. The objective of HAPPIEST was to study the role of aerostatic HAPS in future telecommunications networks, identifying the most promising services and the resulting hybrid networks. In particular, a conceptual design has been developed in two reference scenarios where HAPS services are more valuable from a commercial point of view. This effort enables technical discussion on the technology maturity, economic estimations and programmatic solutions for a future ESA HAPS programme. The setup of a development roadmap for HAPS was another objective of the study.