2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/227729This is the written version of a talk given in memory of Gunnar Källén, at the Departments of Theoretical Physics, Physics, and Astronomy of Lund University on February 13, 2009. It will be published in a collection of the papers of Gunnar Källén, edited by C. Jarlskog and A. C. T. Wu. I discuss some of Källén's work, especially regarding the problem of infinities in quantum field theory, and recount my own interactions with him. In addition, I describe for non-specialists the current status of the problem, and present my personal view on how it may be resolved in the future.16 pages, minor changes in text, many references addedHigh Energy Physics - TheoryGeneral Relativity and Quantum CosmologyHistory and Philosophy of Physicstext