Platforms are widely used to design embedded systems. They have numerous advantages: separation from its application, industrial rationalization, standardization, division of large development teams. However, their design complexity is growing dramatically due to several sources: the intricate combination of parallelism and heterogeneity in modern architectures; the quest for ever low power consumption and the diversity of sensors/actuators required by modern applications. This complexity prevents straightforward platform design in one step and calls for gradual design by composition and improvement over existing components. However, there is no systematic way of composing them, and there is no clear concept suitable for platform composition. In this paper, we propose two atomic ways of composing platforms, increment and assembly, that allow designers to build platforms gradually thanks to two concepts called world and container.