This document summarizes a lecture on perturbative QCD calculations in electron-positron annihilation. It discusses how soft and collinear divergences arise in real emission diagrams but cancel out between real and virtual corrections. It also introduces the concept of jets to define infrared-safe observables and calculates the Sterman-Weinberg jet cross section to next-to-leading order. The lecture emphasizes that perturbative QCD is only well-defined for infrared and collinear safe observables.