This paper presents a unified model for role-based access control (RBAC) developed by NIST. RBAC is a proven technology for large-scale authorization, but the lack of a standard model has caused confusion about its utility and meaning. The NIST model aims to resolve this by unifying ideas from prior RBAC models, commercial products, and research prototypes. It serves as a foundation for developing future standards. RBAC is a rich and evolving technology that allows for the specification and enforcement of various protection policies tailored to enterprise needs. The NIST RBAC model is organized into four levels of increasing functional capabilities: flat RBAC, hierarchical RBAC, constrained RBAC, and symmetric RBAC. These levels are cumulative, with each adding exactly one new requirement. The model also includes an alternate approach that combines flat and hierarchical RBAC in an ordered sequence with two unordered features—constraints and symmetry. Flat RBAC is the simplest form, where users are assigned to roles, and permissions are assigned to roles. It allows for many-to-many user-role and permission-role assignments and enables users to exercise permissions of multiple roles simultaneously. It is widely deployed and familiar, serving as a starting point for RBAC. Hierarchical RBAC introduces role hierarchies, which are partial orders defining a seniority relation between roles. It includes two sub-levels: general hierarchical RBAC, which supports arbitrary partial orders, and limited hierarchical RBAC, which supports simpler structures like trees or inverted trees. Constrained RBAC adds the requirement for enforcing separation of duties (SOD), which spreads responsibility and authority for actions or tasks among multiple users. This helps prevent fraud and accidental damage. Symmetric RBAC adds the requirement for permission-role review, similar to user-role review. This allows for efficient determination of which permissions are assigned to a role and which roles a permission is assigned to. This feature is deferred to level 4 due to its complexity in large-scale systems. The NIST model focuses on aspects of RBAC for which consensus is available. It identifies important attributes of RBAC not included in the model, some of which are not suitable for inclusion in a consensus document. Others require further work and agreement before standardization is feasible. The NIST RBAC model provides a structured approach to RBAC, with four levels of increasing complexity. It aims to provide a standardized framework for RBAC, enabling the development of future standards and promoting interoperability and portability of RBAC tools.This paper presents a unified model for role-based access control (RBAC) developed by NIST. RBAC is a proven technology for large-scale authorization, but the lack of a standard model has caused confusion about its utility and meaning. The NIST model aims to resolve this by unifying ideas from prior RBAC models, commercial products, and research prototypes. It serves as a foundation for developing future standards. RBAC is a rich and evolving technology that allows for the specification and enforcement of various protection policies tailored to enterprise needs. The NIST RBAC model is organized into four levels of increasing functional capabilities: flat RBAC, hierarchical RBAC, constrained RBAC, and symmetric RBAC. These levels are cumulative, with each adding exactly one new requirement. The model also includes an alternate approach that combines flat and hierarchical RBAC in an ordered sequence with two unordered features—constraints and symmetry. Flat RBAC is the simplest form, where users are assigned to roles, and permissions are assigned to roles. It allows for many-to-many user-role and permission-role assignments and enables users to exercise permissions of multiple roles simultaneously. It is widely deployed and familiar, serving as a starting point for RBAC. Hierarchical RBAC introduces role hierarchies, which are partial orders defining a seniority relation between roles. It includes two sub-levels: general hierarchical RBAC, which supports arbitrary partial orders, and limited hierarchical RBAC, which supports simpler structures like trees or inverted trees. Constrained RBAC adds the requirement for enforcing separation of duties (SOD), which spreads responsibility and authority for actions or tasks among multiple users. This helps prevent fraud and accidental damage. Symmetric RBAC adds the requirement for permission-role review, similar to user-role review. This allows for efficient determination of which permissions are assigned to a role and which roles a permission is assigned to. This feature is deferred to level 4 due to its complexity in large-scale systems. The NIST model focuses on aspects of RBAC for which consensus is available. It identifies important attributes of RBAC not included in the model, some of which are not suitable for inclusion in a consensus document. Others require further work and agreement before standardization is feasible. The NIST RBAC model provides a structured approach to RBAC, with four levels of increasing complexity. It aims to provide a standardized framework for RBAC, enabling the development of future standards and promoting interoperability and portability of RBAC tools.