A short introduction to basics of classical and molecular genetics is given focussing on principles of inheritance, DNA structure and DNA function. Evolution and the five evolutionary factors mutation, migration, recombination, drift, selection are introduced. Basics of the analysis of population structure and measurement of genetic diversity within and among populations are described. Human impacts on genetic diversity patterns by breeding, land use change and climate change are outlined and measures to conserve plant genetic resources are discussed. Case studies will mainly deal with genetic diversity patterns of tropical plants.