The IGAC Science Plan and Implementation Strategy lays out the scientific objectives and key research issues of the atmospheric chemistry project of the International Geosphere Biosphere Programme (IGBP) as both IGAC and IGBP enter their second phase. It also lays out a framework for addressing these objectives and issues, recognizing the need for collaboration with partner programmes and projects. The scientific focus of this document emerged from the first decade of IGAC research, much of which was conducted in the context of focused, intensive measurement campaigns. The scope of IGAC in its next phase includes both regional characterisation and the extension into issues that cross more expansive boundaries in space, time and discipline. While local and regional-scale atmospheric chemical composition will be a primary focus, it is now clear that issues such as intercontinental transport and transformation of chemically active species and the interactions between atmospheric chemistry and climate must also be addressed in order to better understand atmospheric chemical composition and to provide guidance to the public and policy-making community.