This review article by ULRICH HEINZ and RAIMOND SNELLINGS discusses the collective flow, its anisotropies, and event-to-event fluctuations in relativistic heavy-ion collisions, focusing on the extraction of the specific shear viscosity of quark-gluon plasma (QGP) from experimental data. The authors highlight the similarities between the Big Bang of the universe and the "Little Bangs" created in heavy-ion collisions, emphasizing the importance of dissipative effects in understanding the QGP dynamics. The article covers several key topics: 1. **Historical Overview**: It traces the development of relativistic fluid dynamics from its early days to the current understanding of QGP viscosity. 2. **Relativistic Heavy-Ion Collision Dynamics**: This section delves into second-order viscous relativistic fluid dynamics, the equation of state (EOS), and the transport coefficients of QGP. 3. **Initial-State Density and Shape Fluctuations**: It discusses the harmonic eccentricity and flow coefficients, centrality classes, and models for the primordial fluctuation power spectrum. 4. **Hydrodynamic Response to Initial-State Fluctuations**: This part explains how viscous effects influence radial and anisotropic flow, and how these effects can be used to extract the QGP shear viscosity. 5. **Transverse Momentum Spectra and Radial Flow**: It explores the systematic variations in radial flow at RHIC and LHC energies, and how these relate to the QGP viscosity. 6. **Elliptic and Other Anisotropic Flow Coefficients**: This section examines the elliptic flow systematics, triangular flow, and flow angle correlations. 7. **Synopsis and Future Perspective**: It provides a comprehensive overview of the progress and future directions in the field. The authors emphasize the importance of event-by-event fluctuations in the initial conditions of heavy-ion collisions, which are crucial for understanding the QGP viscosity and the anisotropic flow coefficients. They also discuss the challenges and recent advancements in experimental techniques and theoretical models, highlighting the need for hybrid approaches that couple hydrodynamic descriptions of the QGP phase with microscopic hadron cascade simulations.This review article by ULRICH HEINZ and RAIMOND SNELLINGS discusses the collective flow, its anisotropies, and event-to-event fluctuations in relativistic heavy-ion collisions, focusing on the extraction of the specific shear viscosity of quark-gluon plasma (QGP) from experimental data. The authors highlight the similarities between the Big Bang of the universe and the "Little Bangs" created in heavy-ion collisions, emphasizing the importance of dissipative effects in understanding the QGP dynamics. The article covers several key topics: 1. **Historical Overview**: It traces the development of relativistic fluid dynamics from its early days to the current understanding of QGP viscosity. 2. **Relativistic Heavy-Ion Collision Dynamics**: This section delves into second-order viscous relativistic fluid dynamics, the equation of state (EOS), and the transport coefficients of QGP. 3. **Initial-State Density and Shape Fluctuations**: It discusses the harmonic eccentricity and flow coefficients, centrality classes, and models for the primordial fluctuation power spectrum. 4. **Hydrodynamic Response to Initial-State Fluctuations**: This part explains how viscous effects influence radial and anisotropic flow, and how these effects can be used to extract the QGP shear viscosity. 5. **Transverse Momentum Spectra and Radial Flow**: It explores the systematic variations in radial flow at RHIC and LHC energies, and how these relate to the QGP viscosity. 6. **Elliptic and Other Anisotropic Flow Coefficients**: This section examines the elliptic flow systematics, triangular flow, and flow angle correlations. 7. **Synopsis and Future Perspective**: It provides a comprehensive overview of the progress and future directions in the field. The authors emphasize the importance of event-by-event fluctuations in the initial conditions of heavy-ion collisions, which are crucial for understanding the QGP viscosity and the anisotropic flow coefficients. They also discuss the challenges and recent advancements in experimental techniques and theoretical models, highlighting the need for hybrid approaches that couple hydrodynamic descriptions of the QGP phase with microscopic hadron cascade simulations.