Marketing measurement and optimization helps B2C marketers update and develop programs as market conditions change. Today, data is analyzed, and insights are derived continuously, enabling a more modern planning approach than the front-loaded planning process that determines marketing strategies months in advance. To keep pace with changing conditions, B2C marketers must migrate to an agile planning process that allows them to quickly shift marketing strategies. This report outlines the guardrails needed for this flexible marketing planning process. This is an update of a previously published report; Forrester reviews and revises it periodically for continued relevance and accuracy.