Whether labeling and advertising claims for multi-ingredient dietary supplements may be based on the testing of individual, key ingredients--rather than the actual product--has caused a good deal of confusion. This confusion stems from the dearth of case law and the open-endedness of Federal Trade Commission (FTC) and Food and Drug Administration (FDA) guidance on this issue. Nevertheless, the relevant regulatory guidance, case law and self-regulatory case law--when assessed together--indicate that the law allows and even protects "key ingredient claims" (i.e., claims based on efficacy testing of key ingredients in the absence of full product testing). This article provides an overview of the relevant substantiation requirements for dietary supplement claims and then reviews FTC's and FDA's guidance on key ingredient claims; relevant case law; use of key ingredient claims in the advertising of other consumer products; and the National Advertising Division of the Better Business Bureau, Inc.'s (NAD's) approach to evaluating key ingredient claims for dietary supplements. This article concludes that key ingredient claims--provided they are presented in a truthful and non-deceptive manner--are permissible, and should be upheld in litigation and cases subject to industry self-regulation. This article further concludes that the NAD's approach to key ingredient claims provides practical guidance for crafting and substantiating dietary supplement key ingredient claims.