A car platform is a shared set of common design, engineering, and production efforts, as well as major components, over a number of outwardly distinct models and even types of cars, often from different, but somewhat related, marques. It is practiced in the automotive industry to reduce the costs associated with the development of products by basing those products on a smaller number of platforms. This further allows companies to create distinct models from a design perspective on similar underpinnings. A car platform is not to be confused with a platform chassis, although such a chassis can be part of an automobile's design platform, as noted below.

A basic definition of a platform in cars, from a technical point of view, includes underbody and suspensions (with axles) — where the underbody is made of the front floor, rear floor, engine compartment, and frame (reinforcement of underbody). Key mechanical components that define an automobile platform include:

Platform sharing is a product development method where different products and the brand attached share the same components. The purpose with platform sharing is to reduce the cost and have a more efficient product development process. The companies gain on reduced procurement costs by taking advantage of the commonality of the components. However, this also limits their ability to differentiate the products and imposes a risk of losing the tangible uniqueness of the product. The companies have to make a trade-off between reducing their development costs and the degree of differentiation of the products.

Platform sharing is a practice commonly employed by various brands within a corporate group. The fundamental components of a shared platform typically include the chassis and the drive unit. The extent to which different automobile or motorcycle models utilize the same components can vary, leading to different degrees of structural equality and platform similarity:

The remaining vehicle parts are categorised into "head" parts and system parts:

Platform sharing facilitates the efficient production and development of vehicles by leveraging common components across different models, thereby reducing costs and enhancing operational efficiency.

One of the first car companies to use this product development approach was General Motors in 1908. General Motors used a single chassis for certain class of model across most of its brands like Chevrolet, Buick, Pontiac and Oldsmobile. Later, Chrysler Corporation would do the same for Plymouth, DeSoto and Dodge cars. Ford followed the same principle for Ford and Mercury in US markets. The chassis unit was common with many shared mechanical components while the exterior styling and interior trims were designed according to its individual brand and category.

In recent years for monocoque chassis, platform-sharing combined with advanced and flexible-manufacturing technology enabled automakers to sharply reduce product development and changeover times, while modular design and assembly allow building a greater variety of vehicles from one basic set of engineered components. Pictured below is the Nissan MS platform, where designs including 5-door hatchback, sedan, compact SUV and minivan were built on a common floor panel and many shared functional assemblies such as engine, transmission and chassis components.