The l1 periodogram searches for periodicities in unevenly sampled time series. It can be used similarly as a Lomb-Scargle periodogram, and retrieves a figure which has a similar aspect but has fewer peaks due to aliasing. It is primarily designed for the search of exoplanets in radial velocity data, but can be also used for other purposes. The principle of the algorithm is to search for a representation of the input signal as a sum of a small number of sinusoidal components, that is a representation which is sparse in the frequency domain. Here, "small number" means small compared to the number of observations.