This course module is the first one to focus on a core subject of building biology, which is how to create a comfortable and healthy indoor climate. Only if the various indoor climate factors – such as air, temperature, humidity, and electroclimate – are working properly together, the desired comfort can be achieved. This course module is an introduction and also provides an overview of this complex issue. Many of the issues raised here will be discussed in more detail in subsequent course modules including “Building Materials and Building Science,” “Heating and Ventilation,” “Energy-efficient Building and Retrofitting,” “Electromagnetic Radiation,” “Pollutants and Mold,” “Light and Lighting,” and “Natural Colors and Finishes.”