The period-luminosity relation (PLR) is an important relationship between the fundamental physical properties of many types of variable stars. Most recognisable is the PLR of Cepheids, which allows classical Cepheids to be used as primary distance indicators, establishing the traditional cosmic distance scale. Well-defined relationships also exist between the periods and luminosities of many other types of pulsating stars. IAU Symposium 376 discusses recent observational and theoretical results achieved by the continued, detailed scrutiny of the PLR, including its shape, calibration and dependence on stellar parameters and chemical abundances. Intrinsic PLR widths may offer unique insights into the physical processes shaping these relations and the underlying physical properties of the stars contributing to these relationships. This topic is timely, given the wealth of data recently obtained using state-of-the-art ground- and space-based facilities, high-impact Gaia data releases and the initial operations of the James Webb Space Telescope.