Since air‐ or spaceborne imaging usually records how an object or a scene interacts with solar radiant energy (amongst many other interactions such as those taking place in the atmosphere and inside the camera), it is advisable to start our journey with a concise exploration of the world of electromagnetic energy. This entry will give some highly simplified insights into the properties of electromagnetic radiation and explain how they are harnessed when creating a digital image from a terrestrial or aloft platform. These insights should serve as essential building blocks for future entries. Since the latter will focus on the imaging pipeline of a digital camera and tackle related concepts such as spatial, spectral and radiometric resolution, they will build upon the concepts introduced here.