This review presents a comprehensive overview of microscopic models for ultrarelativistic heavy ion collisions, focusing on the UrQMD model. The paper discusses the theoretical foundations of transport theory in nuclear collisions, including non-relativistic and relativistic transport theories, as well as the Quantum Molecular Dynamics (QMD) approach. It highlights the importance of understanding the dynamics of heavy ion collisions across a wide range of energies, from the Coulomb barrier to ultra-relativistic energies, and how these collisions can probe the properties of hot and dense nuclear matter. The paper emphasizes the role of transport theory in interpreting experimental results and predicting new phenomena in relativistic heavy ion collisions. It discusses the limitations of current theoretical models in describing the full range of collision dynamics, particularly at high energies where quark and gluon degrees of freedom become significant. The UrQMD model is introduced as a microscopic transport model that incorporates various reaction mechanisms, including particle and resonance production, string excitation and fragmentation, and color fluctuations. The review also explores the implications of heavy ion collisions for understanding phase transitions in nuclear matter, such as the transition to a quark-gluon plasma (QGP) and the restoration of chiral symmetry. It discusses the importance of experimental observables like baryon stopping, collective flow, and particle production in probing these phenomena. The paper highlights the challenges in interpreting experimental data due to the complexity of the collision dynamics and the need for a detailed understanding of the underlying physics. The review concludes by emphasizing the importance of transport theory in describing the time evolution of heavy ion collisions and the need for further theoretical and experimental work to refine our understanding of the dynamics of these collisions. It also notes the limitations of current models and the need for more accurate descriptions of the self-energies and cross sections in transport theory. The paper underscores the significance of the UrQMD model in providing a framework for understanding the complex dynamics of heavy ion collisions and the potential for future developments in this field.This review presents a comprehensive overview of microscopic models for ultrarelativistic heavy ion collisions, focusing on the UrQMD model. The paper discusses the theoretical foundations of transport theory in nuclear collisions, including non-relativistic and relativistic transport theories, as well as the Quantum Molecular Dynamics (QMD) approach. It highlights the importance of understanding the dynamics of heavy ion collisions across a wide range of energies, from the Coulomb barrier to ultra-relativistic energies, and how these collisions can probe the properties of hot and dense nuclear matter. The paper emphasizes the role of transport theory in interpreting experimental results and predicting new phenomena in relativistic heavy ion collisions. It discusses the limitations of current theoretical models in describing the full range of collision dynamics, particularly at high energies where quark and gluon degrees of freedom become significant. The UrQMD model is introduced as a microscopic transport model that incorporates various reaction mechanisms, including particle and resonance production, string excitation and fragmentation, and color fluctuations. The review also explores the implications of heavy ion collisions for understanding phase transitions in nuclear matter, such as the transition to a quark-gluon plasma (QGP) and the restoration of chiral symmetry. It discusses the importance of experimental observables like baryon stopping, collective flow, and particle production in probing these phenomena. The paper highlights the challenges in interpreting experimental data due to the complexity of the collision dynamics and the need for a detailed understanding of the underlying physics. The review concludes by emphasizing the importance of transport theory in describing the time evolution of heavy ion collisions and the need for further theoretical and experimental work to refine our understanding of the dynamics of these collisions. It also notes the limitations of current models and the need for more accurate descriptions of the self-energies and cross sections in transport theory. The paper underscores the significance of the UrQMD model in providing a framework for understanding the complex dynamics of heavy ion collisions and the potential for future developments in this field.