The "Plasma Experiment" aboard of the solar probe HELIOS consists of four independent instruments which are designed to investigate the interplanetary plasma, the so-called solar wind. Primarily the velocity distribution function of the different kinds of particles are measured. All important hydrodynamic parameters of the solar wind plasma can then be derived. Three instruments analyse the positive component of the solar wind (protons and heavier ions with energy-per-charge values from 0.155 to 15.32 kV). Two of them permit an angular resolution in both directions of incidence. One instrument measures electrons in the energy range from 0.5 to 1660 eV with a one-dimensional angular resolution. Since the launch all the instruments, which are partially novel developments, have performed very well.