not intended to portray a linear, sequential process, begins with a value-chain and lifecycle perspective. It depicts sources of nanomaterials originating throughout the lifecycle and value chain, and therefore the environmental or physiologic context that these materials are embedded in, and the processes that they affect. The circle, identified as “critical elements of nanomaterial interactions,” represents the physical, chemical, and biologic properties or processes that are considered to be the most critical for assessing exposure and hazards and hence risk. Those elements exist on many levels of biologic organization, including molecular, cellular, tissue, organism, population, and ecosystem. The committee asks, What are the most important elements that one would examine to determine whether a nanomaterial is harmful? and has placed these elements at the center of the proposed research framework. The critical elements in the circle are not ordered, and the dynamic interactions among them are implied. For example, factors that affect surface affinity may also affect persistence and bioaccumulation and would not be appropriately reflected in any linear sequencing of the elements. Research needs relating to such critical elements are discussed in Chapter 3. Research priorities for addressing the critical elements are summarized in Chapter 5.