This publication presents the Proceedings of the 6th Seminar of the Subnetwork on Production Systems of the FAO-CIHEAM Network for Research and Development in Sheep and Goats, held in Ponte de Lima (Portugal) from 1 5th to 1 7th November 2007, with the title of: "Changes in Sheep and Goat Farming Systems at the Beginning of the 21st Century". The Seminar was organised by FAO, CIHEAM, Direcçâo Regional de Agricultura e Pescas do Norte (DRAP-Norte), and the Escola Superior Agrâria de Ponte de Lima (ESAPL) - Instituto Politécnico de Viana do Castelo (IPVC). The Seminar dealt with the diversification of sheep and goat farming, the sustainability of the present production systems, the possible coexistence between wild animals and smail ruminants, and the evolution of the product quality and its impact on the production systems. It also reviewed and assessed the progress made on subjects analysed and discussed during previous seminars of the Subnetwork, such as methods for evaluating these systems, including recent methods such as modelling. The final objective was to provide elements for reflection, results of the actions already undertaken and the new methodologies for colleagues responsible for the development of the sheep and goat sector, in their activities to improve the livestock sector and its adaptation to the new challenges. This volume collects the edited texts of 47 communications presented in the Meeting and two additional articles that review the progress made on the analysis, evaluation and management of production systems.