Why Are There Unintended Consequences of Program Action, and What Are the Implications for Doing Evaluation?

Reasons for unintended consequences of program action are explained. Although this topic has been dealt with extensively in the field of planning, that literature provides little guidance for how evaluation methodologies can be improved. Implications for evaluation are provided by drawing the distinction between consequences that are unforeseen and consequences that are unforeseeable and then recommending tactics for dealing with each.