The sequencing of the Drosophila genome allowed the identification of most coding sequences, highlighting the necessity for a functional assignation of the identified genes. The information extracted from the sequence directly classified a considerable fraction of genes into known molecular categories, although there is still a large proportion of them that, due to poor sequence conservation, are not included into any informative class. Furthermore, in many instances the molecular nature of a protein is not particularly revealing about its functional requirements and network of interactions. In this manner, complementary genomic approaches to gene identification by sequence conservation are fundamental both in Drosophila and other organisms to assign particular functions to annotated genes. The approach more successful in the Drosophila field is the undertaking of genetic screenings to identify sets of interacting genes and genes controlling particular cellular processes. Classic genetic screens comprise all those based on a “phenotypic” paradigm, where the generation of large collections of mutant chromosomes is followed by their mapping. This approach has been recently expanded to include “genomic” tools, such as the use of microarrays and interference RNA, as well as reverse-genetics techniques, seeding the way to a “functional” annotation of the Drosophila genome.Peer reviewed