Probes and amplification systems represent new technologic developments in the domain of clinical microbiology. In spite of their high specificity, simple applicability and high velocity the DNA probes show only little sensitivity especially for the direct detection of microorganisms in clinical samples. This sensitivity lead to the development of DNA-amplification technics. PCR is the most famous of them and the most often used one. Next to the direct detection of microorganisms in clinical samples without culture these technics also allow the direct isolation of bacterial toxins, of genes for resistance to antibiotics as well as a systematic search for pathogens in diseases with unknown cause. Different problems of these methods in daily clinical practice are illustrated by four applications 1. direct detection of mycobacterium tuberculosis complex (M. tbc) in clinical samples, 2. rapid identification of mycobacterial species, 3. their typing for epidemiologic intentions, 4. rapid detection of resistance to antibiotics.