It was in December 1900 when Max Planck found the quantization of an electromagnetic field in explaining the spectrum of the blackbody radiation. With his quantum hypothesis Max Planck started a revolution in physics. The quantum theory developed afterwards in the first part of the last century is today the basis of our modern technology. Despite the fact that quantum phenomena play an important role in technology there are still conceptional problems with the theory which are, of course, not based on the formal concept, but on the philosophical foundation. The step between quantum physics describing microscopic systems and the classical macroscopic world is simply too big. In recent years there have been a series of experiments with single atoms and photons which facilitate the understanding of quantum phenomena and simultaneously lead to interesting new applications such as the single atom laser and trapped ions for quantum information processing and frequency standards; these will be the main subject of the talk.