Radioisotope techniques, to the naive, may represent a miracle in scientific research the answer to all investigational difficulties. On the other hand, to the cynic they may be considered as a fad which creates more problems than it solves. The truth seems to be somewhere between the two extremes. The real importance of these techniques are recognized by workers who are well grounded in their fields and who have been involved in the application of these techniques. They are able to recognize the important problems in their own fields, are familiar with the experimental material, and should be able to interpret results of their experiments. Nevertheless, it is only fair to recognize the many difficulties in handling, obtaining and storing radioisotopes. This paper is a brief discussion of some of the fundamentals of this research tool which has developed in recent years and some of its applications in plant research. It may give some ideas as to the possibilities of this technique in attacking some of the problems in agriculture in general and in plant research in particular. A brief review of some chemical and physical concepts familiar to all may be of some interest. All matter is composed of discrete particles, atoms, of the elements, or of larger particles, molecules, made up of combinations of these elementary atoms. Further, that "isotopes" are atoms of an element having the same chemical properties as all other atoms of the same element but differing slightly in weight. In nature a few of these isotopes are radioactive that is they are unstable and spontaneously charge into atoms of another (or the same) element with radiation of energy in the form-gamma rays, or alpha or beta particles. With the advent of controlled nuclear reactions, it has become possible to produce radioactive isotopes which do not occur in nature, by the exposure of selected elements to neutrons. Limited quantities of such "synthetic" elements had been produced earlier by the bombard-ment of specific target materials by other particles such as protons or alpha particles but the yields were small and the operation expensive. These atoms have some readily recognized characteristics: The radiator given off by, and the time required for disintegration of, the unstable atoms is characteristic of the particular isotope. For example, P32, the important radioisotope of phosphorus, emits only beta particles with a maximum energy of 1.69 Mev and decays at a rate such that onehalf of the initial quantity remains after 14.3 days and each succeeding 14,3 days interval reduces the amount remaining by 50% (a "halfIife" of 14.3 days) while the isotope of, S35 gives off betas of only 1/10 the energy and has a half life of 87.1 days. C14, particularly useful in the study of biological systems, has a half life of 5568 years; emitting beta particlesof 0.155 Mev. The alpha particles, beta particles, and gamma rays emitted by the disintegrating atoms are "ionizing radiations", that is they have the common ability to produce electrons and positive ions in matter through which they pass. These pairs of oppositely charged ions are formed at the expence of energy loss from incident radiation. Instruments for the detection and measurement of ionization are extremely sensitive, the entrance of a single alpha particle into the "sensitive volume" of a modern counter can be readily detected. Since the radiation arising from the decay of radioactive atoms permits the detection of individual decaying atoms of individual decaying atoms of the element, it is possible to make quantitative measurements of extremely small quantities. For example, with good counting methods one can conveniently measure 2 X 10 - 11 brams of radioactive carbon. This extreme sensitivity of detection combined with the circumstance that the radio-active elements behave in chemical or biological systems as do their inactive counterparts, provide a technique for obtaining data hitherto an attainble and of unusual scope, the "Tracer" Technique". We can now get answers to such questions as: What proportion of a nutrient added to the soil is utilized by the crop? Is a hormone or growth regulator localized or widely translocated? And, more important perhaps, we have a means of making essentially direct observation on the fundamental or basic reactions of biological systems. by: Prof. Dr. Aly M. Lasheen;Guru besar Plant Physiology Fakultas Pertanian Universitas Indonesia Bogor.