Filamentous fungi include a large, heterogeneous group of heterotrophic organisms with profound influence in human activities. In spite of their economic and scientific relevance, little information is available at the molecular level about their biology. The development of genetic transformation protocols has contributed greatly to the molecular dissection of fungal behavior. The use of this approach in combination with large-scale genome sequencing projects has now provided the basis for gaining insight into the function of fungal genes. This chapter reviews the technology of the transformation of filamentous fungi. The protocols for gene transference by protoplasting/PEG, LiAc, electroporation, biolistics and A. tumefaciens are described, and possible mechanisms for transformation are discussed. A brief description of previously reported selection systems is also included. The application of transforming protocols concerning the study of gene function and manipulation are discussed in relation to some fungal species.