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The paper presents first a review of the work on phenolic resins done by Euler and his co‐workers, by Hultzsch and by Zinke and his team, and of the importance of this work for the knowledge of the hardening process. More recent research work of Zinke and his co‐workers, especially on cyclic polynuclear phenols is also described. There is a short discussion of the chemistry of novolaks and of the work of Ziegler. Results of research on the hexamine‐hardening process are also discussed.