The variable relevant for the impact of sea level rise on coastal areas is relative sea level or, as is preferred here, LSL. At any coastal location, LSL is influenced by a number of processes with spatial scales from local to global. Each of these processes has its own characteristic spatial and temporal scales. With respect to future changes, each process is associated with its own Probability Density Function (PDF), which in most cases is geographically and temporally variable. Establishing a combined PDF for LSL at a given location will therefore be extremely difficult if not impossible. What can be done is to consider projection of plausible trajectories of future sea level for a wide range of forcing scenarios, similar to the approach taken by the Intergovernmental Panel for Climate Change (IPCC) for the assessment of future climate change. However, this approach requires detailed knowledge of the relation between forcing on a wide range of spatial and temporal scales and LSL at a specific location. Consequently, in order to provide projections of future LSL, it is necessary to understand the forcing mechanisms of LSL variations and secular change.