This chapter clarifies the terminology and the definitions assigned to various concepts that revolve around the imperfection and uncertainty of geographic information. These terms have been used in different ways over the years. The chapter introduces the different parts of this book while also revealing which issues they tackle. The first part of the book describes the foundations and main concepts related to the imperfection of geographic data. The issue is to shed light on and provide a summary of terminologies, the origins of imperfections, as well as the concepts of quality, integrity, and confidence. The second part of the book tackles the main representations of imperfection and their applications for geographic information. The final part of the book introduces a few data processing and reasoning systems that involve spatial objects. Imperfection is considered in relation to our knowledge about the objects.