The plasma monitoring system HERCULES utilizes the Self Excited Electron Plasma Resonance Spectroscopy (SEERS) technique. It takes into account the non-linearity of the space charge sheath at the rf electrode, which provides harmonics with the modulated sheath width and high frequency oscillations in the bulk plasma. By using a general discharge model, SEERS provides volume averaged values of electron collision rate, electron density, and bulk power. It thus provides a very efficient real time data compression. The rf sensor head at the chamber walls is on ground potential and does not influence plasma- or process conditions. The sensor measures rf currents only, hence, there is no impact of polymer or other insulating layers on the measured signals. HERCULES is the first process control tool providing real-time accessibility to plasma parameters - based on an electrical measurement principles - for rf plasmas under industrial conditions. The efficient data handling being ready for use in production includes an internal process data bank and offers two ways to control the process: 1) independent of the etch tool, and 2) using a data coupling utility providing fast and easy access to wafer and lot data. Here, the second possibility was used based on a chamber log-sheet for lot and single wafer control. The major benefit of this choice is that data can be analyzed by lot and product using standard software. The SEERS tool demonstrated a high sensitivity and significant correlation of measured signals to variations of fundamental process parameters. It was found to be well suited to control stability of process equipment and to support process optimization and development. Important suppliers, e.g. Applied Materials and Lam Research, have supported the assessment and are interested in applying this process control tool.