Notes on QED Corrections in Weak Decays

In these lecture notes the basics of QED corrections to hadronic decays are reviewed with special emphasis on conceptual (e.g. counting and tracking of infrared sensitive logs) rather than numerical aspects. General matters are illustrated for the cases of increased complexity and decreased inclusiveness: $e^+ e^- \to hadrons$, the leptonic decay $\pi^+ \to \ell^+ \bar \nu$ and the semileptonic decay $B \to \pi \ell^+ \bar \nu$. The non-trivial and ongoing efforts of including structure dependence are very briefly outlined.