Seed is an essential input for crop production. For farmers, access to affordable and high quality seeds is therefore important for increasing agricultural production and productivity. In many developing countries, the formal seed system has shifted from a publicly dominated sector to a privately dominated sector. Nevertheless, the largest share of seeds are still produced by farmers themselves. In this Knowledge Portal topic, the focus is on seed sector systems and technical seed system innovations. Specific focuses are: integrated seed sector development, linking of informal and formal seed systems, balance between public- and private-sector involvement, seed quality and quality control, and seed production.