Genetic engineering of algae is not common due to problems related to the design of vectors (i.e. plasmids or viruses) that can be successfully incorporated into the algae, accepted by the cell and expressed in a satisfying way. Most studies have therefore been made on the "green yeast" Clamydomonas reinhartii and some cyanobacterial species. However, in this review we are presenting examples of studies performed on a broad collection of algal species ranging from cyanobacteria to macroalgae like Laminaria. We have included different kinds of applications, within physiology, biochemistry, molecular biology, phylogeny, industry and environmental science. This ongoing and forthcoming research will undoubtedly increase our knowledge and usage of these important and fascinating primary-producing organisms.