This chapter defines evaluation and impact assessment and discusses five challenges facing event evaluators and impact assessors. Complexity in terms of theory, politics and methodology increases as evaluation tasks move from problem solving and decision support to the assessment of cumulative impacts from event portfolios and attainment of long-term sustainability. The first challenge is to differentiate the concepts of merit and worth and how they can be evaluated. The second challenge is to establish a system for permanent and comprehensive evaluation and impact assessment. The use of logic and theory of change models is then recommended, a process requiring multi-stakeholder collaboration. The fourth challenge involves the need for standardization, the absence of which has rendered numerous evaluations and impact studies incomparable and lacking credibility. Finally, there is a need for transparency and accountability in all evaluation and impact assessment processes. Both theory development and professional practice will be advanced by addressing these challenges.