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Software as a service (SaaS /sæs/^[1]) is a cloud computing service model where the provider offers use of application software to a client and manages all needed physical and software resources.^[2] SaaS is usually accessed via a web application. Unlike other software delivery models, it separates "the possession and ownership of software from its use".^[3] SaaS use began around 2000, and by 2023 was the main form of software application deployment.

Unlike most self-hosted software products, only one version of the software exists^{[citation needed]} and only one operating system and configuration is supported. SaaS products typically run on rented infrastructure as a service (IaaS) or platform as a service (PaaS) systems including hardware and sometimes operating systems and middleware, to accommodate rapid increases in usage while providing instant and continuous availability to customers. SaaS customers have the abstraction of limitless computing resources, while economy of scale drives down the cost. SaaS architectures are typically multi-tenant; usually they share resources between clients for efficiency, but sometimes they offer a siloed environment for an additional fee. Common SaaS revenue models include freemium, subscription, and usage-based fees. Unlike traditional software, it is rarely possible to buy a perpetual license for a certain version of the software.

There are no specific software development practices that distinguish SaaS from other application development, although there is often a focus on frequent testing and releases.