A Commentary on Teichmüller's paper ``Untersuchungen über konforme und quasikonforme Abbildungen" (Investigations on conformal and quasiconformal mappings) (to appear in Vol. VII of the Handbook of Teichmüller theory).

This is a commentary on Teichmüller's paper Untersuchungenüber konforme und quasikonforme Abbildungen (Investigations on conformal and quasiconformal mappings) published in 1938. The paper contains fundamental results in conformal geometry , in particular a lemma, known as the Modulsatz, which insures the almost circularity of certain loci defined as complementary components of simply connected regions in the Riemann sphere, and another lemma, which we call the Main Lemma, which insures the circularity near infinity of the image of circles by a quasiconformal map. The two results find wide applications in value distribution theory, where they allow the efficient use of moduli of doubly connected domains and of quasiconformal mappings. Teichmüller's paper also contains a thorough development of the theory of conformal invariants of doubly connected domains.The final version of this paper will appear in Vol. VII of the \emph{Handbook of Teichmüller theory} (European Mathematical Society Publishing House, 2020)