In this introductory chapter, first the nature of petroleum fluids, hydrocarbon types, reservoir fluids, crude oils, natural gases, and petroleum fractions are introduced and then types and importance of characterization and physical properties are discussed. Application of materials covered in the book in various parts of the petroleum industry or academia as well as organization of the book are then reviewed followed by specific features of the book and introduction of some other related books. Finally, units and the conversion factors for those parameters used in this book are given at the end of the chapter.