Bringing the data and new findings in ecology to bear on ecosystems assessment, which can contribute to the protection and reasonable use, and can make human wellbeing progress toward sustainable development. Based on the first product of “Millennium Ecosystem Assessment”, this paper has presented main concepts and its implications of ecosystem assessment in detail. Because the emphasis of ecosystem assessment is to study the relationship between ecosystem services and human wellbeing, the main concepts of ecosystem assessment expounded in this paper include ecosystem and its services, human wellbeing and poverty reduction, drivers of ecosystem change, crossscale interaction and assessment of ecosystems, and values associated with ecosystem. Successful ecosystem assessment is vitally dependent upon the understanding of the above concepts. Presently, we are facing challenges on carrying on integrated assessment of ecosystem services，driving force analysis, and scaling issues.