Sky surveys produce enormous quantities of data on extensive regions of the sky. The easiest way to access this information is through catalogues of standardised data products. XMM-Newton has been surveying the sky in the X-ray, ultra-violet and optical bands for 20 years. The XMM-Newton Survey Science Centre has been producing standardised data products and catalogues to facilitate access to the serendipitous X-ray sky. 4XMM-DR12 contains 939270 X-ray detections above the processing likelihood threshold of 6. These X-ray detections relate to 630347 unique X-ray sources. A significant fraction of sources (117124, 19%) have more than one detection in the catalogue (up to 74 repeat observations in the most extreme case).