This paper analyses the integration process that firms follow to implement Supply Chain Management (SCM). This study has been inspired in the integration model proposed by Stevens (1989). He suggests that companies internally integrate first and then extend integration to other supply chain members, such as customers and suppliers. To analyse the integration process a survey was conducted among Spanish food manufacturers. The results show that there are companies in three different integration stages. In stage I, companies are not integrated. In stage II, companies have a medium-high level of internal integration in the Logistics-Production interface, a low level of internal integration in the Logistics-Marketing interface, and a medium level of external integration. And, in stage III, companies have high levels of integration in both internal interfaces and in some of their supply chain relationships.