High throughput screening has rapidly evolved during the last 15 years. Many assay technologies have been created to allow rapid and efficient screening for most drug discovery targets.most of the use technologies have now become matures. Radioactivity was mostly used during the early nineties; then fluorescence became the most used mean to measure binding, enzymatic activities or cellular physiology. Fluorescence resonance energy transfer (FRET), fluorescence polarization (or anisotropy) and of course fluorescence intensity are very often used; they are also suitable for miniaturization. nowadays label-free technologies are also coming on the market. It has also begun very important to monitor the effects of drug substances on cell physiology; therefore high content screening has emerged as an essential tool during the early drug discovery phase.