Willard Libby visited Lindau and lectured two times, first at the physics meeting in 1971 and then at the chemistry meeting 1974. This is symptomatic, since his activities as radio-chemist really bridged the gap between physics and chemistry. Each element in the periodic system has isotopes, some of which are radioactive. Since most isotopes have the same chemical properties as the corresponding stable element, radioactive isotopes can be used as tracers in biological systems. Signals from radioactive decay can then, e.g., show the path taken by the stable element through the human body. This idea worked out and gave George de Hevesy the 1943 Nobel Prize in Chemistry. At around the same time Willard Libby realized that the cosmic radiation hitting the atmosphere produces a radioactive isotope of stable carbon designated carbon -14. This means that when living matter absorbs carbon dioxide, CO2, a certain fraction is radioactive. When the living matter stops absorbing, e.g., when a tree or an animal dies, this fraction starts to diminish by radioactive decay. But this means that by measuring the fraction of carbon-14 still left today, the age of the remnants of the tree or animal can be determined. But, as explained by Libby in his talk, it is not really that easy! One problem is that the amount of cosmic radiation may have changed over time so that sometimes the living matter received a higher or a lower fraction of carbon-14. In order to surmount this problem, one can try to compare with other dating methods such as the year rings of trees or the layered sediments at the bottom of lakes. Libby describes how he with radio-chemical methods has been able to put a date on matter about 7 500 years old and predicts that he will reach 10 000 years. Today physicists use accelerators to count the number of radioactive atoms left in the specimen and the limit has been extended to about 50 000 years.