<p>The Solar and Heliospheric Observatory (SOHO) was launched on 2 December 1995. An international collaboration between ESA and NASA, the mission was designed to study the Sun from its deep core to the outer corona and the solar wind. Thirty years after its launch, SOHO has provided a nearly continuous record of solar and heliospheric phenomena for more than a full 22-year magnetic cycle. Its observations have revolutionized our understanding of the Sun and heliosphere, and the mission is still producing high-quality data on a daily basis. SOHO has produced images of structures and flows below the Sun’s surface, and of activity on the far side of the Sun. SOHO data eliminated uncertainties in the internal structure of the Sun as a possible explanation for the ‘solar neutrino problem’. SOHO provided evidence for the upward transfer of magnetic energy from its surface to the corona and identified the source regions of the fast solar wind. Furthermore, it has pioneered modern space weather forecasting capabilities. This Review aims to synthesize the scientific legacy of the mission, highlighting key discoveries and its role in spawning a series of new space missions and inspiring a whole generation of scientists.</p>