This Notice of Funding Opportunity (NOFO) encourages applications for investigator-initiated, early phase, clinical trials of natural products (i.e., botanicals, probiotics, and products marketed as dietary supplements), which have a strong scientific premise to justify further clinical testing. For this NOFO, natural products include promising nutritional regimens that standardize the amount of a specific naturally occurring nutritional compound (e.g., omega-3 fatty acids, anthocyanidins, or polyphenols) and have compelling preliminary evidence. Under this NOFO, trials must be designed so that results, whether positive or negative, will provide information of high scientific utility and will support decisions about further development or testing of the natural product.This NOFO will provide up to three years (R61 phase) of support for milestone-driven testing of pharmacokinetics, bioavailability, and assessment of the natural products effect (i.e., measure of mechanism of action) when used by humans on a specified target measure. If milestones in the R61 phase are achieved, up to 3 years of additional support (R33 phase) may be awarded to replicate the impact of the natural product on target engagement(s) when used by humans and assess whether there is an association between the degree of the impact on the target engagement and clinical outcomes in a participant population. Applications are encouraged to design R33 studies to determine how to optimize the impact of the natural product on the target engagement by optimizing the delivery of the natural product through examination of different doses or formulations. In addition, applications can be designed to combine the natural product with another treatment approach that is known to impact the same target engagement measure; or study the impact of the natural product in a population that is more responsive,