Indoor climate is determined by rational lighting, heating, cooling and ventilating systems. For occupants' well-being it should be consistent with how regional outdoor climate works in the flow of radiation via four paths of heat transfer: radiation; convection; conduction; and evaporation. This book starts with the relationship between the human body and its immediate environmental space followed by a brief introduction of passive and active systems for indoor climate conditioning. The nature of light and heat is discussed with a focus on building envelope systems such as walls and windows, and then examined from the viewpoint of thermodynamics and human-biology. Some examples are given to enable a better understanding of luminous and thermal characteristics of our most immediate environment particularly for those professionally involved in environmental planning, designing, and engineering to know about bio-climatic design principle.