This chapter introduces the fundamental concepts of Qualitative Process (QP) theory, a framework for reasoning about physical processes and quantities. The theory aims to capture common-sense physical reasoning by defining a simple notion of physical processes and representing quantities using quantity spaces. Quantity spaces are partial orders that describe the relationships between different values of a quantity, allowing for qualitative reasoning about changes over time. The chapter also introduces individual views, which describe the contingent existence and states of objects, and functional relationships, which capture the dependencies between quantities. The chapter concludes with an overview of the basic concepts and their applications in various physical scenarios, such as modeling fluids, motion, and oscillators.This chapter introduces the fundamental concepts of Qualitative Process (QP) theory, a framework for reasoning about physical processes and quantities. The theory aims to capture common-sense physical reasoning by defining a simple notion of physical processes and representing quantities using quantity spaces. Quantity spaces are partial orders that describe the relationships between different values of a quantity, allowing for qualitative reasoning about changes over time. The chapter also introduces individual views, which describe the contingent existence and states of objects, and functional relationships, which capture the dependencies between quantities. The chapter concludes with an overview of the basic concepts and their applications in various physical scenarios, such as modeling fluids, motion, and oscillators.