The chapter introduces the theoretical framework for understanding nuclear collisions, focusing on transport theory and microscopic models. It begins with a brief overview of the importance of studying nuclear matter under extreme conditions, highlighting the motivation for investigating the equation of state and phase transitions. The chapter then delves into the non-relativistic and relativistic transport theories, detailing the derivation of transport equations from effective Lagrangians and the role of Green's functions. It discusses the limitations of these theories, such as the quasi-particle approximation and the need for realistic G-matrices. The chapter also covers the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, which is a microscopic transport model used to describe heavy ion collisions. The model is detailed in Chapter 3, and its application to various phenomena, including baryon stopping, collective flow, and particle production, is discussed in Chapter 4. The chapter concludes with a summary and conclusions, emphasizing the importance of linking experimental observables to different phases of nuclear matter.The chapter introduces the theoretical framework for understanding nuclear collisions, focusing on transport theory and microscopic models. It begins with a brief overview of the importance of studying nuclear matter under extreme conditions, highlighting the motivation for investigating the equation of state and phase transitions. The chapter then delves into the non-relativistic and relativistic transport theories, detailing the derivation of transport equations from effective Lagrangians and the role of Green's functions. It discusses the limitations of these theories, such as the quasi-particle approximation and the need for realistic G-matrices. The chapter also covers the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, which is a microscopic transport model used to describe heavy ion collisions. The model is detailed in Chapter 3, and its application to various phenomena, including baryon stopping, collective flow, and particle production, is discussed in Chapter 4. The chapter concludes with a summary and conclusions, emphasizing the importance of linking experimental observables to different phases of nuclear matter.