This article describes, analyses, and reflects upon the challenges of planning and conducting the evaluation of the KidsMatter Initiative (KMI) in Australian primary schools during the first year of the two-year initiative. The purpose of the evaluation is to inform the Australia-wide rollout of the KMI. The discussion is arranged under four headings: (a) conceptual challenges, (b) design challenges, (c) practical implementation challenges, and (d) managing collaborative complexity. Emphasis is placed on how the challenges were theorised as a basis for resp onding to the requirements of the evaluation. Conclusions include recognising and enabling contributions from diverse stakeholders; utilising the domain expertise of the evaluation team; operating flexibly to meet the needs and exigencies of the KMI, the clients, an d the diverse participant groups; maintaining focus upon the core conceptual frameworks underlying the KMI and the evaluation; and implications of the evaluation for developing wider knowledge relating to schools and their effects, as well as about factors contributing to educational change.