Abstract In this paper an overview of the helical advanced stellarator (Helias) reactor studies is presented. The Helias reactor (HSR) is a reactor candidate of the optimized stellarator line, developed at the Max-Planck Institut für Plasmaphysik, in Germany. It is an up-scaled version of the Wendelstein 7X, which accommodates a blanket and a shield and combines the modular coils with the optimized properties of the fusion plasma [T. Andreeva, C.D. Beidler, E. Harmeyer, Yu.L. Igitkhanov, Ya.I. Kolesnichenko, V.V. Lutsenko, A. Shishkin, F. Herrnegger, J. Kißlinger, H. Wobig, The Helias reactor concept: comparative analysis of different field period configurations, Fusion Sci. Technol. 46 (2004) 395–400]. A compact reference version is HSR4/18 with 4-field periods and a 18 m major radius. HSR4/18 can stably operate at an average beta 4–5% and to deliver a fusion power of 3 GW. The design of the HSR demonstrates criteria of a safe and reliable route to ignition, a self-sustained burn without external heating, reliability of the technical components and the tritium breeding blanket. The coil system, the vacuum vessel and the shielding blanket are discussed. A more compact HSR3/15 configuration with 3-field periods and a 15 m major radius shows, however, a considerable amount of the bootstrap current and requires further optimization. A quasi-isodynamical configuration without transitional particle orbits was considered as an option, which confines almost 100% energetic particles.