Stellar winds are ubiquitous among massive stars. They play a crucial role in stellar evolution, and strongly affect observed spectra. While it is generally accepted that the winds are driven by radiation pressure, acting through bound‐bound transitions, many details remain unclear. Observational and theoretical evidence suggests that the winds are clumped, which greatly complicates the interpretation of spectra. More recently, the accuracy with which we can predict mass‐loss rates has been questioned. Some lines of evidence suggest that mass‐loss rates are lower than theoretical values by a factor of a few. More controversial is evidence that in some stars the mass‐loss rates are an order of magnitude, or more, lower than expected. In this review we briefly discuss our current understanding of the fundamental parameters of O stars. We discuss the many different line formation mechanisms that operate in stellar winds. We also summarize the wide variety of techniques that can be used to infer and constrain current, and in some cases historic, wind properties.