The formation of complicated nuclei from constitutents such as protons and neutrons. According to the big bang models, the early universe was filled with a particle soup of protons and neutrons. Nucleosynthesis includes all processes by which, from these humble beginnings, arose the complex atomic nuclei that we find in the universe today.

The first light elements (mainly nuclei of deuterium, helium, and lithium) formed already at cosmic time between a few seconds and a few minutes (primordial nucleosynthesis or Big Bang Nucleosynthesis); more massive nuclei up to those of iron formed and continue to form in the course of fusion processes inside stars; nuclei that are even more massive form in the course of supernova explosions. These explosions also serve to disseminate the complex nuclei formed inside stars (stellar nucleosynthesis) in space.

A brief account of Big Bang Nucleosynthesis can be found in the spotlight text Big Bang Nucleosynthesis, while Equilibrium and change provides more information about the physical processes involved and Elements of the past describes how the predictions of Big Bang Nucleosynthesis can be tested against astronomical observation.