Delivery of nicotine in the most desirable form is critical in maintaining people's use of tobacco products. Interpretation of results by tobacco industry scientists, studies that measure free-base nicotine directly in tobacco smoke, and the variability of free-base nicotine in smokeless tobacco products all indicate that the form of nicotine delivered to the tobacco user, in addition to the total amount, is an important factor in whether people continue to use the product following their initial exposure. The physiological impact of nicotine varies with the fraction that is in the free-base form and this leads to continued exposure to other toxic tobacco contents and emissions. In addition to evaluating the total nicotine delivered to the user, measuring the fraction of nicotine in the free-base form is critical in understanding and controlling the influence of nicotine on tobacco use.