To understand the complex functioning of the climate system and its evolution under the effect of natural and anthropogenic forcings, as well as the impacts of climate change on natural systems and human societies, major international organizations and space agencies in many countries have recommended or set up, for about three decades, a wide variety of observation systems for climatic parameters, at global, regional and local scales. The GCOS (Global Climate Observing System) has defined dozens of essential climate variables (ECVs) that must be observed very precisely over the long term, from space or the ground, to better understand the processes involved and their interactions, and validate models simulating future evolutions. Sea level from global to global scale is one of them. Closure of the sea level budget at global and regional scales is also part of the GCOS goals. For studies based on the global mean and regional sea level budget, we need sustained sea level observations from satellite altimetry with quasi global coverage. This means continuity of the high-precision altimetry record beyond the Sentinel-6 Michael Freilich mission, reprocessing of past missions and coverage the Arctic Ocean; For the steric component, it is crucial to maintain Core Argo and implement Deep Argo, perform regular subsurface temperature and salinity measurements in marginal seas, high latitudes, boundary currents, and shallow areas and shelf regions; perform systematic calibration of Argo data using independent observing systems; For the mass component, we also need sustained measurements of ocean mass changes, of ice sheet and glaciers mass balances, and of land water storage changes from GRACE-type missions with improved performances. Sustained monitoring of land ice bodies using other remote sensing systems (InSAR, radar and optical imagery, standard radar as well as SAR/SARIN, and laser altimetry) and modeling are also requested; Improvement of global hydrological models is also a major goal to estimate the land water component. This requires more accurate global digital elevation models as well as remote-sensing-based stream flow estimates. In addition to observational requirements, we also recommend continuing modeling efforts for terms not yet easily accessible by observations; e.g., GIA and fingerprints of present-day land ice melt. Another major challenge concerns coastal sea level. SAR technology and dedicated reprocessing of classical altimetry missions now allows to estimate sea level changes in the world coastal zones, with sometimes significant departure from open ocean changes. Unfortunately, coastal data (e.g., in situ temperature and salinity, coastal currents, river discharge, bathymetry, shoreline position, etc.) needed to quantify small-scale coastal processes that superimpose to the global and regional factors, are very sparse or inexistent in most of the world coastal zones. A specific effort is crucially needed to implement coastal observation networks, especially along vulnerable shorelines, that combine a variety of sensors from space and ground (including GNSS collocated tide gauges) to improve knowledge of forcing factors on coastal sea level and associated impacts. Development of very-high resolution hydrodynamical models covering coastal zones is also highly recommended.