In software engineering, multitier architecture (often referred to as n-tier architecture) is a client–server architecture in which presentation, application processing and data management functions are physically separated. The most widespread use of multitier architecture is the three-tier architecture.

N-tier application architecture provides a model by which developers can create flexible and reusable applications. By segregating an application into tiers, developers acquire the option of modifying or adding a specific tier, instead of reworking the entire application. A three-tier architecture is typically composed of a presentation tier, a logic tier, and a data tier.

While the concepts of layer and tier are often used interchangeably, one fairly common point of view is that there is indeed a difference. This view holds that a layer is a logical structuring mechanism for the conceptual elements that make up the software solution, while a tier is a physical structuring mechanism for the hardware elements that make up the system infrastructure. For example, a three-layer solution could easily be deployed on a single tier, such in the case of an extreme database-centric architecture called RDBMS-only architecture or in a personal workstation.