Register modernisation is a core project of public sector digitalisation, aiming to digitally interconnect existing government data holdings such as population, trade, or vehicle registers. In Germany, the Register Modernisation Act enables the use of the tax identification number as a universal identifier across authorities. The once-only principle ensures citizens provide data just once, while agencies retrieve it directly from registers. Key challenges include harmonising heterogeneous data schemas, ensuring data protection compliance, and technically integrating legacy systems. Successful register modernisation is a prerequisite for fully digital public services and significantly reduces administrative burden for both citizens and government employees.