Welcome to the repository for the Hierarchical Engine for Large-scale Infrastructure Co-Simulation (HELICS). HELICS provides an open-source, general-purpose, modular, highly-scalable co-simulation framework that runs cross-platform (Linux, Windows, and Mac OS X). It is not a modeling tool by itself, but rather an integration tool that enables multiple existing simulation tools (and/or multiple instances of the same tool), known as "federates," to exchange data during runtime and stay synchronized in time such that together they act as one large simulation, or "federation". This enables bringing together established (or new/emerging) off-the-shelf tools from multiple domains to form a complex software-simulation without having to change the individual tools (known as "black-box" modeling). All that is required is for someone to write a thin interface layer for each tool that interfaces with existing simulation time control and data value updating, such as through an existing scripting interface. Moreover, the HELICS community has a growing ecosystem of established interfaces for popular tools, such that many users can simply mix and match existing tools with their own data and run complex co-simulations with minimal coding. Today the core uses of HELICS are in the energy domain, where there is extensive and growing support for a wide-range of electric power system, natural gas, communications and control-schemes, transportation, buildings, and related domain tools. However, it is possible to use HELICS for co-simulation in any domain. Previous and existing HELICS efforts have stretched across a wide range of scales in time and space: From transient dynamics (e.g. power system frequency response or electromechanical transient simulation) through steady-state power flow and markets to long-term planning studies. And from individual appliance behaviors, through distribution & bulk systems, and to nation-wide simulations.