The electron-proton collider HERA, like an electron-mycroscope, explores the structure of the proton down to10−16 cmand up to the situation of very high parton densities. The proton energy was upgraded from 820 to 920 GeV in the Fall of ’98 and the luminosity has also substantially improved, with another factor of 3 upgrade expected to follow this year. Inclusive proton structure functions have been studied with incidente+ande−of 27 GeV in the neutral (NC) and charged (CC) current interactions as functions of the squared four-momentum transfer,Q2,and of the fractional proton momentum carried by partons,x. The structure functionF2,as well as the&ggr;−Z0interference termxF3,have been measured in a range ofQ2and1/xthat extends by orders of magnitude that reached by fixed target experiments. The DGLAP evolution equations [1] allow for a perturbative NLO QCD fit of the measured non-perturbative structure functions in the available kinematic range:&agr;Sand the gluon density at lowxare fitted at the same time with good precision. The longitudinal structure function,FL,can be determined within the DGLAP formalism. With CC, the electroweak unification has been tested; at highx, a first flavor decomposition of the light quarks is achieved. The contribution toF2of the charm quark has been measured and results to be relevant. Bounds on the radius of quarks and on compositeness are derived from the data at the highestQ2,100<Q2⩽40,000 GeV2.©2001 American Institute of Physics.