  Measurements of proton structure functions made in neutral and charged
current interactions at HERA are discussed, covering four-momentum transfers
Q^2 from about 0.5 GeV^2 to 30000 GeV^2. The results include the rise of the
structure function F2 towards small x and electro-weak effects at high Q^2. QCD
fits made by the H1 and ZEUS collaborations provide both, parton densities with
uncertainties, and precise alpha_s determinations.
