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In modern physics, and perhaps this was true earlier, there are two different trends. One group of physicists has the aim of investigating new laws and solving existing contradictions. They believe the result of their work to be a theory; in particular, the creation of the mathematical apparatus of modern physics. As a by-product there appear new principles for constructing devices, physical devices.The other group, on the contrary, seeks to create physical devices using new physical principles. They try to avoid the inevitable difficulties and contradictions on the way to achieving that purpose. This group considers various hypotheses and theories to be the by-product of their activity.Both groups have made outstanding achievements. Each group creates a nutrient medium for the other and therefore they are unable to exist without one another; although, their attitude toward each other is often rather critical. The first group calls the second "inventors," while the second group accuses the first of abstractness or sometimes of aimlessness. One may think at first sight that we are speaking about experimenters and theoreticians. However, this is not so, because both Copyright ? 1965 by the Nobel Foundation. The author is on the staff of the Lebedev Institute in Moscow, U.S.S.R. This article is the lecture he delivered in Stockholm, Sweden, 11 December 1964, when he received the Nobel Prize in Physics, a prize which he shared with A. M. Prokhorov, also of the Lebedev Institute, and Charles H. Townes, of the Massachusetts Institute of Technology. The lecture is published here with the permission of the Nobel Foundation and will also be included in the complete volumes of Nobel lectures in English, published by the Elsevier Publishing Company, Amsterdam and New York. 20 AUGUST 1965