As the need for accountable marketing spending continues to grow, companies must develop sound metrics and measures of marketing’s contribution to firm profitability. The leading metric has been return on marketing investment (MROI), following the widespread adoption of ROI metrics in other parts of the organisation. However, the ROI metric in marketing is typically interpreted and used in a variety of ways, which causes ambiguity and suboptimal marketing decision making. This paper seeks to remove the ambiguity around MROI to guide better measurements and analytics aligned to financial contribution. The authors first provide a formal definition of MROI and review variations in the use of MROI that are the root cause of ambiguity in interpretation. The authors come to the conclusion that MROI estimates would be more transparently described if those providing the estimates used the following form: Our analysis measured a (total, incremental, or marginal) MROI of (scope of spending) using (valuation method) over time period. The paper proceeds to describe five case studies that illustrate the various uses of MROI, covering different marketing initiatives in different business sectors. The authors describe the important links between marketing lift metrics (such as response elasticities) and MROI. The final section of the paper focuses on the connection between MROI and business objectives. While management’s prerogative is to maximise short- and long-run profits, that is not equivalent to maximising MROI. The authors demonstrate that MROI plays a different role in the process of marketing budget setting (a marketing strategic task) versus allocating a given budget across different marketing activities (a marketing operations task). They highlight the role of setting MROI hurdle rates that recognise not only marketing’s ability to drive revenue, but also the firm’s cost of capital. The authors hope that their recommendations will help the marketing profession achieve a common understanding of how to assess and use what they believe is its most important summary productivity metric, MROI.