In April 2007, more than 60 evaluators, funders, state leaders, and national experts came together in Pittsburgh for a symposium on evaluating systems initiatives — efforts to build or reform health, education, or human service systems for purposes of improving individual or community well-being. The symposium was sponsored by The Build Initiative, a foundation-funded multi-state effort to ensure that children from birth through age fi ve are safe, healthy, eager to learn, and ready to succeed in school. 2 The Build Initiative supports states' efforts to build comprehensive and coordinated early childhood systems of programs, policies, and services that work together to achieve positive outcomes for young children and their families. 3 Evaluating systems efforts like The Build Initiative in ways that both capture their impact and inform their ongoing development can be a signifi cant challenge. Systems initiatives are complex and notoriously "hard to measure." They involve multiple programs and players and feature outcomes at multiple levels (individual, family, community, and state). They involve numerous public or private funding streams administered through different agencies and decision-making structures. They require aligning goals and coordinating actions across programs with different political cultures. And, they tackle diffi cult deep-rooted problems such as gaps in services and outcomes based on race, income, culture, and language. Finally, all efforts to improve systems are long-term efforts that evolve over time in response to the political, economic, and social contexts around them. These many complexities place systems initiatives directly outside of the more familiar and more traditional program evaluation comfort zone. Consequently, less consensus exists about how to assess them. Still, systems initiative evaluation is not uncharted territory. Systems initiatives have been around for decades and various evaluation approaches have been tried. Of particular note are "theory of change" evaluation approaches that have gained substantial momentum since the mid 1990s when the Aspen Institute's Roundtable on Comprehensive Community Initiatives for Children and Families introduced them as a promising approach for evaluating complex initiatives. Theories of change are now the cornerstone of many, if not most, systems initiative evaluations. 4 But while theories of change have added much to evaluation practice in this area, they are not (and did not promise to be) a panacea for all evaluation dilemmas that systems initiatives present. . In practice they have been more a way of describing system elements and systems initiative complexities than an evaluation methodology that spells out initiative assumptions and ways of testing whether they are valid.