The Annual Review of Fluid Mechanics presents a comprehensive overview of slip flow at the atomistic level in dilute gases and dense liquids. Slip flow occurs when the tangential velocity of a fluid differs from that of the solid boundary it contacts. This phenomenon is crucial for understanding fluid dynamics at small scales and is described by slip boundary conditions that replace the traditional no-slip condition. Slip flow is particularly important in nanoscale and microscale applications, where it enhances flow rates and can significantly affect the response to driving forces. In dilute gases, slip flow is governed by kinetic theory and is described by the Boltzmann equation. The slip length, a measure of the distance over which the fluid velocity profile extrapolates to the wall velocity, is influenced by factors such as the degree of surface accommodation and the gas–solid interaction model. The slip boundary condition is derived from the inhomogeneity introduced by the solid boundary and is typically expressed as a linear relationship between the fluid velocity and the slip length. In dense liquids, slip flow is dominated by intermolecular interactions, and the internal scale for hydrodynamic purposes is the atomic/molecular size. Slip in liquids is influenced by the structure and properties of the liquid, and the presence of a solid boundary can lead to significant variations in local properties. The slip boundary condition in liquids is similarly described by a linear relationship between the fluid velocity and the slip length, but the slip length is strongly dependent on the gas–solid interaction model. The review discusses the similarities and differences between slip in gases and liquids, the characterization and measurement of slip using molecular simulations, and models for predicting slip. It also highlights open questions in the field, such as the need for more accurate and predictive methods for calculating slip coefficients and the importance of understanding slip flow in bridging the gap between different length scales. The review concludes with a summary of the current understanding of slip flow in dilute gases and dense liquids, emphasizing the importance of slip flow in both scientific and practical contexts. It also outlines future research directions, including the development of more accurate models and the application of slip flow theory to a wider range of systems and conditions.The Annual Review of Fluid Mechanics presents a comprehensive overview of slip flow at the atomistic level in dilute gases and dense liquids. Slip flow occurs when the tangential velocity of a fluid differs from that of the solid boundary it contacts. This phenomenon is crucial for understanding fluid dynamics at small scales and is described by slip boundary conditions that replace the traditional no-slip condition. Slip flow is particularly important in nanoscale and microscale applications, where it enhances flow rates and can significantly affect the response to driving forces. In dilute gases, slip flow is governed by kinetic theory and is described by the Boltzmann equation. The slip length, a measure of the distance over which the fluid velocity profile extrapolates to the wall velocity, is influenced by factors such as the degree of surface accommodation and the gas–solid interaction model. The slip boundary condition is derived from the inhomogeneity introduced by the solid boundary and is typically expressed as a linear relationship between the fluid velocity and the slip length. In dense liquids, slip flow is dominated by intermolecular interactions, and the internal scale for hydrodynamic purposes is the atomic/molecular size. Slip in liquids is influenced by the structure and properties of the liquid, and the presence of a solid boundary can lead to significant variations in local properties. The slip boundary condition in liquids is similarly described by a linear relationship between the fluid velocity and the slip length, but the slip length is strongly dependent on the gas–solid interaction model. The review discusses the similarities and differences between slip in gases and liquids, the characterization and measurement of slip using molecular simulations, and models for predicting slip. It also highlights open questions in the field, such as the need for more accurate and predictive methods for calculating slip coefficients and the importance of understanding slip flow in bridging the gap between different length scales. The review concludes with a summary of the current understanding of slip flow in dilute gases and dense liquids, emphasizing the importance of slip flow in both scientific and practical contexts. It also outlines future research directions, including the development of more accurate models and the application of slip flow theory to a wider range of systems and conditions.