A transposable element (TE) is a DNA sequence that can change its relative position (self-transpose) within the genome of a single cell. The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create phenotypically significant mutations and alter the cell's genome size. Barbara McClintock's discovery of these jumping genes early in her career earned her a Nobel prize in 1983.

TEs make up a large fraction of the C-value of eukaryotic cells. They are often considered "junk DNA". In Oxytricha, which has a unique genetic system, they play a critical role in development. They are also very useful to researchers as a means to alter DNA inside a living organism.

Transposable elements are only one of several types of mobile genetic elements. They are assigned to one of two classes according to their mechanism of transposition, which can be described as either "copy and paste" (for class I TEs) or "cut and paste" (for class II TEs).

Class I (retrotransposons): They copy themselves in two stages, first from DNA to RNA by transcription, then from RNA back to DNA by reverse transcription. The DNA copy is then inserted into the genome in a new position. Reverse transcription is catalyzed by a reverse transcriptase, which is often coded by the TE itself. Retrotransposons behave very similarly to retroviruses, such as HIV.