Transposable elements are one of the most important players in genome evolution. The most obvious contribution of TEs to evolution is gene duplication, which provides new genetic material on which mutation, drift, and selection can act. There are several mechanisms involving TEs by which gene duplication can occur. These mechanisms can be divided into two main groups: DNA-based duplication and RNA-based duplication. In DNA-based duplication and transduction, the duplicated region can contain part of a gene, an entire gene, or multiple genes. In the latter two cases, promoters and introns may be conserved and present in duplicated genes. This is in sharp contrast to the results of RNA-based retroposition, where mature mRNA is retrotranscribed into cDNA and duplicated genes are intronless and stripped of regulatory sequences. There is increasing evidence that these ‘dead on arrival’ retrocopies can acquire active promoters and become functional. The evolutionary fate of duplicated genes varies. Many of them accumulate mutations and become pseudogenized. Others can either retain the function of their ancestors or undergo subfunctionalization or neofunctionalization. Gene duplications can have direct phenotypic consequences and contribute significantly to the ability of species to adapt to changing environments, but they are also associated with many human diseases.