Measuring the efficacy of ESG has been in the works for decades. While investors have long relied on globally accepted benchmarks such as Generally Accepted Accounting Principles, or GAAP, to compare the financial performance of companies, no such standard exists in ESG. Yet this is changing as organizations reach consensus on common means for reporting greenhouse gas emissions, as well as other ESG criteria. To be sure, responsible investors are still dealing with a dizzying array of data sources, acronyms, and jargon. But there has been progress! This chapter lays out the key groups and concepts driving the measurement of ESG investing, which is vital to the credibility of the approach.