Food security and nutrition can only be assured if it is based on sustainable production practices and crop seeds of good quality and purity. Good quality seeds are needed on farm as a major asset to boost crop yields. However, availability of good quality seeds largely depends on investment by the private sector, which depends on availability of a significant market to ensure profit. But, enforcement of different national seed legislations, which is the case in many countries, for example in Africa, does not provide a seed market large enough to attract investment from the private sector. In this context, the Food and Agriculture Organization of the United Nations (FAO) encourages and supports regional harmonization of rules governing quality control, certification and marketing of crop seeds. While regional harmonization enlarges the seed market and makes it attractive for investment by the private sector, care must be taken to upgrade the seed standards by insertion of the list of dangerous seed borne pests, pathogens and weeds, to anticipate on their possible spread over entire regions. With regard to weeds, contamination of crop seeds by weed propagules is an important issue in several countries in Africa, Asia and Latin America. Contamination of crop seeds by weed propagules contributes to spreading of weeds to places where they were not encountered before, which increases weed management cost incurred by the farmer, may take land out of production and may become an impediment to sustainable crop production intensification.