The QCD phase diagram and Beam Energy Scan physics: a theory overview This chapter reviews recent theoretical developments relevant to heavy-ion experiments conducted in the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). The focus is on describing the dynamics of systems created in heavy-ion collisions and establishing the connection between experimental observables and the QCD phase diagram. The QCD phase diagram is a key area of research, with the goal of determining the equation of state (EOS) of QCD quantitatively. The BES program aims to explore the QCD phase diagram by varying the beam energy, which is the crucial experimental control parameter. The QCD phase diagram is a complex structure that includes the transition between the Quark-Gluon Plasma (QGP) and Hadron Resonance Gas (HRG) phases. The phase diagram is explored through lattice calculations, which can reliably calculate the EOS at finite temperature and zero chemical potential, but face challenges at finite baryon density due to the sign problem. Other theoretical approaches, such as the Functional Renormalization Group (FRG) and the Nambu-Jona-Lasinio (NJL) model, also contribute to understanding the phase diagram. Heavy-ion collisions provide a way to explore the QCD EOS and phase diagram. By colliding heavy nuclei at various energies, researchers can scan the range of temperatures and baryon chemical potentials where the QCD phase transition occurs. The BES program aims to determine the location of the QCD critical point, which is the point where the phase transition becomes discontinuous. This critical point is important for understanding the phase structure of QCD. The chapter discusses the multistage description of bulk dynamics in heavy-ion collisions, including the prehydrodynamic stage, hydrodynamics with multiple conserved charges, freeze-out, and hadronic afterburner. It also covers the microscopic transport description of dense nuclear matter dynamics, including the equation of state and fluctuations. The chapter highlights the importance of understanding the dynamics of heavy-ion collisions, including the initial stage, hydrodynamic evolution, and the role of fluctuations in determining the QCD phase diagram. The chapter concludes with a summary of the key findings and future directions in the study of the QCD phase diagram and Beam Energy Scan physics. It emphasizes the importance of connecting theoretical models with experimental observations to better understand the QCD phase diagram and the behavior of matter under extreme conditions.The QCD phase diagram and Beam Energy Scan physics: a theory overview This chapter reviews recent theoretical developments relevant to heavy-ion experiments conducted in the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). The focus is on describing the dynamics of systems created in heavy-ion collisions and establishing the connection between experimental observables and the QCD phase diagram. The QCD phase diagram is a key area of research, with the goal of determining the equation of state (EOS) of QCD quantitatively. The BES program aims to explore the QCD phase diagram by varying the beam energy, which is the crucial experimental control parameter. The QCD phase diagram is a complex structure that includes the transition between the Quark-Gluon Plasma (QGP) and Hadron Resonance Gas (HRG) phases. The phase diagram is explored through lattice calculations, which can reliably calculate the EOS at finite temperature and zero chemical potential, but face challenges at finite baryon density due to the sign problem. Other theoretical approaches, such as the Functional Renormalization Group (FRG) and the Nambu-Jona-Lasinio (NJL) model, also contribute to understanding the phase diagram. Heavy-ion collisions provide a way to explore the QCD EOS and phase diagram. By colliding heavy nuclei at various energies, researchers can scan the range of temperatures and baryon chemical potentials where the QCD phase transition occurs. The BES program aims to determine the location of the QCD critical point, which is the point where the phase transition becomes discontinuous. This critical point is important for understanding the phase structure of QCD. The chapter discusses the multistage description of bulk dynamics in heavy-ion collisions, including the prehydrodynamic stage, hydrodynamics with multiple conserved charges, freeze-out, and hadronic afterburner. It also covers the microscopic transport description of dense nuclear matter dynamics, including the equation of state and fluctuations. The chapter highlights the importance of understanding the dynamics of heavy-ion collisions, including the initial stage, hydrodynamic evolution, and the role of fluctuations in determining the QCD phase diagram. The chapter concludes with a summary of the key findings and future directions in the study of the QCD phase diagram and Beam Energy Scan physics. It emphasizes the importance of connecting theoretical models with experimental observations to better understand the QCD phase diagram and the behavior of matter under extreme conditions.