With the completion of the Arabidopsis thaliana genome sequencing project the next major challenge is the assignment of biological functions to the more than 25,000 genes. Reverse genetics is a powerful tool to elucidate gene function in Arabidopsis. Increasingly sophisticated genetic approaches are being developed for reverse genetics with the long-term goal of understanding how the coordinated activity of all proteins rises to a complex organism. Identification of a biological function for each gene is often doomed to fail as many loss- or gain-of-function lines exhibit no obvious phenotypes under normal propagation conditions . Here we provide an overview on how to phenotype plants during their development. This phenotypic streamlined analysis is based on a series of defined growth stages (germination, seedling, vegetative, and reproductive stages), which can be used for the profiling of mutants.