This chapter reviews recent theoretical developments relevant to heavy-ion experiments within the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). The primary focus is on describing the dynamics of systems created in heavy-ion collisions and establishing the necessary connection between experimental observables and the Quantum Chromodynamics (QCD) phase diagram. QCD is a non-abelian gauge theory that describes the strong interaction using fundamental principles. However, extracting theoretical predictions from QCD is challenging due to the non-perturbative nature of the strong interaction phenomena, such as quark and gluon confinement and spontaneous chiral symmetry breaking. The exploration of the QCD phase diagram involves several theoretical approaches, including lattice calculations, microscopic transport models, and gravitational wave observations from neutron star mergers. Heavy-ion collisions at various beam energies allow for the exploration of the QCD phase diagram, particularly the transition between the Quark-Gluon Plasma (QGP) and the Hadron Resonance Gas (HRG). The BES program aims to determine the QCD equation of state (EOS) quantitatively by varying the beam energy, which translates into a scan of the QCD phase diagram. The dynamics of heavy-ion collisions are described using multistage hydrodynamic simulations, including the initial collision, hydrodynamic expansion, transition to particles, and hadronic rescatterings. At lower collision energies, microscopic degrees of freedom play a significant role, and the initial conditions are more complex due to the long overlap time between colliding nuclei. The chapter discusses the use of parametric initial conditions, which involve supplementing transverse distributions with longitudinally parametrized profiles. These initial conditions are used in subsequent hydrodynamic evolution and help identify longitudinal profiles from experimental data. The chapter also covers dynamical initialization approaches, where the transition to hydrodynamics occurs at a specified time, and the application of these methods in various contexts, including core-corona interactions and mini-jet studies. The chapter explores the hydrodynamic framework that accounts for the dynamic evolution of multiple conserved charges (baryon number, electric charge, and strangeness). It derives the conservation equations and dissipative equations of motion, providing a clear insight into the underlying physics. The connection between microscopic phase-space distribution and macroscopic hydrodynamic fields is established through kinetic theory, and the dissipative components of the energy-momentum tensor and charge currents are derived. The chapter concludes by discussing the challenges and recent advances in understanding the QCD phase diagram and the role of heavy-ion collisions in exploring it. It emphasizes the importance of rapidity-dependent measurements and the need for a comprehensive understanding of the initial state to interpret experimental data.This chapter reviews recent theoretical developments relevant to heavy-ion experiments within the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). The primary focus is on describing the dynamics of systems created in heavy-ion collisions and establishing the necessary connection between experimental observables and the Quantum Chromodynamics (QCD) phase diagram. QCD is a non-abelian gauge theory that describes the strong interaction using fundamental principles. However, extracting theoretical predictions from QCD is challenging due to the non-perturbative nature of the strong interaction phenomena, such as quark and gluon confinement and spontaneous chiral symmetry breaking. The exploration of the QCD phase diagram involves several theoretical approaches, including lattice calculations, microscopic transport models, and gravitational wave observations from neutron star mergers. Heavy-ion collisions at various beam energies allow for the exploration of the QCD phase diagram, particularly the transition between the Quark-Gluon Plasma (QGP) and the Hadron Resonance Gas (HRG). The BES program aims to determine the QCD equation of state (EOS) quantitatively by varying the beam energy, which translates into a scan of the QCD phase diagram. The dynamics of heavy-ion collisions are described using multistage hydrodynamic simulations, including the initial collision, hydrodynamic expansion, transition to particles, and hadronic rescatterings. At lower collision energies, microscopic degrees of freedom play a significant role, and the initial conditions are more complex due to the long overlap time between colliding nuclei. The chapter discusses the use of parametric initial conditions, which involve supplementing transverse distributions with longitudinally parametrized profiles. These initial conditions are used in subsequent hydrodynamic evolution and help identify longitudinal profiles from experimental data. The chapter also covers dynamical initialization approaches, where the transition to hydrodynamics occurs at a specified time, and the application of these methods in various contexts, including core-corona interactions and mini-jet studies. The chapter explores the hydrodynamic framework that accounts for the dynamic evolution of multiple conserved charges (baryon number, electric charge, and strangeness). It derives the conservation equations and dissipative equations of motion, providing a clear insight into the underlying physics. The connection between microscopic phase-space distribution and macroscopic hydrodynamic fields is established through kinetic theory, and the dissipative components of the energy-momentum tensor and charge currents are derived. The chapter concludes by discussing the challenges and recent advances in understanding the QCD phase diagram and the role of heavy-ion collisions in exploring it. It emphasizes the importance of rapidity-dependent measurements and the need for a comprehensive understanding of the initial state to interpret experimental data.