Platform as a service (PaaS) or application platform as a service (aPaaS) or platform-based service is a cloud computing service model where users provision, instantiate, run and manage a modular bundle of a computing platform and applications, without the complexity of building and maintaining the infrastructure associated with developing and launching application(s), and to allow developers to create, develop, and package such software bundles.

PaaS can be delivered in three ways:

The first public platform as a service was Zimki, launched by Fotango, a London-based company owned by Canon Europe. It was developed in 2005, had a beta launch in March 2006 and a public launch at EuroOSCON in 2006.

At the time of its closure, Zimki had several thousand developer accounts. It had demonstrated the technical viability of Platform as a Service, but also provided the first example of the perils of being dependent on a single provider. This was highlighted when the CEO (Simon Wardley, known for Wardley maps) announced at OSCON 2007 that Zimki would no longer be open-sourced and discussed the future of what was then called framework-as-a-service (later called platform-as-a-service) covering the importance of a market of providers based on an open-source reference model.

The original intent of PaaS was to simplify the writing of code, with the infrastructure and operations handled by the PaaS provider. Originally, all PaaSes were in the public cloud. Because many companies did not want to have everything in the public cloud, private and hybrid PaaS options (managed by internal IT departments) were created.

PaaS provides an environment for developers and companies to create, host and deploy applications, saving developers from the complexities of infrastructure (setting up, configuring and managing elements such as servers and databases). PaaS can improve the speed of developing an app, and allow its user to focus on the application itself. With PaaS, the customer manages applications and data, while the provider (in public PaaS) or IT department (in private PaaS) manages runtime, middleware, operating system, virtualization, servers, storage and networking. Development tools provided by the vendor are customized according to the needs of the user. The user can choose to maintain the software, or have the vendor maintain it.

PaaS offerings may also include facilities for application design, application development, testing and deployment, as well as services such as team collaboration, web service integration, and marshalling, database integration, security, scalability, storage, persistence, state management, application versioning, application instrumentation, and developer community facilitation. Besides the service engineering aspects, PaaS offerings include mechanisms for service management, such as monitoring, workflow management, discovery and reservation.

The advantages of PaaS are primarily that it allows for higher-level programming with dramatically reduced complexity; the overall development of the application can be more effective, as it has built-in/self up-and-down ramping infrastructure resources; and maintenance and enhancement of the application is thus easier.