Discussion of the ionosphere requires a basic knowledge of the upper atmosphere. The reason is that the ionosphere is the partially ionized plasma region that co-exists with the uppermost layers of a planet or moon's atmosphere. This Lecture proceeds by discussing Earth's atmosphere, describing the general physics of ionospheres, and then applying the results to Earth's ionosphere. The Lecture concludes with descriptions of ionospheric outflow of plasma, ionospheric scintillations, and waves in the ionosphere.