Transgenic fish were first made more than 30 years ago. Since then a variety of methods and const ructs have been tested for introducing genetic sequences into fish for scientitlc investigations as well as commercial purposes. Here we review transposable clements and t heir applications in fish. Transposons can be used to deliver genes to chromosomes to confer new traits or as insertional agents and traps to uncover the functions and expression patterns of natural genes in chromosomes. Two DNA transposons have been characterized for transposon-based gene transfer and insertional mutagenesis. The t1rst is d1e Sleeping Beauty transposon system that was reconstituted trom a Tel/ ma1·iner-like relic in salmonid genomes after more than a 10 million year evolutionary sleep. The second is a naturally occurring transposon trom medaka, the Tol2 transposon that belongs to the hAT family of mobile clements. In comparison wid1 random integration of plasmid sequences and pseudotyped retroviral genomcs, transposons have several advantages for genetic studies in fish. These include int roduction of a single, dctlned DNA sequence into a cellular chromosome, stable expression trom the integrant for multiple generations, no absolute size restrictions on d1c transferred gene, case in construction and usc, and safety. Early experiments have validated the versatility of d1c Sleeping Becmt)• transposon for all of these purposes. The applications of transposon systems surpass usc just in fish ; the Sleeping Bcatt/J)' transposon system is being used in mice to discover functions of genes and is being developed for gene therapy in humans.