The use of genetics in plant biology aims at the physiological and molecular genetical characterization of the phenotypic variation for the trait under study. Efficient mutant and gene isolation procedures have been developed for a number of plant models such as Arabidopsis thaliana. For this, the map position of the genes and insertion mutagenesis are used. The latter also allows the characterization of genes that are not easily recognized in mutant approaches, by using enhancer or gene-trapping procedures and reverse genetics. In addition to mutants, natural variation present among wild and cultivated varieties of a species provides an important source of genetic variation. The use of molecular markers, advanced mapping populations and specific cytogenetic stocks in case of polyploids, enables a detailed characterization of such natural variation even when it is of a quantitative and polygenic nature. Examples of the various genetic approaches are given.