Ecological systems are organized across a range of space and time scales. This scaling must be explicitly considered in any attempt to link ecological and atmospheric models. This chapter outlines the elements of Hierarchy Theory which are particularly relevant to this scaling problem. Results are presented as a set of nine criteria. These criteria must be considered before processes, operating on independent scales, can be accurately interfaced to make predictions at the global scale. Hierarchy Theory is proposed as a critical element in developing an understanding of global change.