As cultural and scientific objects, the planets have been understood differently by people across cultures and time periods. The physical understanding of the planets, their origins, natures, movements, and relationship to Earth has changed considerably over time. An historical examination of these changes, beginning with a few examples from the ancient world and then focusing primarily on the European and American contexts from the medieval world to the present, illustrates that the planets are windows into the worldviews and preoccupations of the culture in which planetary ideas and understandings originated. There have also been changes to how historians understand the practice and purpose of planetary observations and theories during these periods, and these changes are addressed in each section.