**1. Introduction**

Sea level rise is primarily an issue at the boundary line with land. It represents a potential threat to infrastructures and population living in low-elevation coastal areas [1]. The land disappears not only because the rising sea changes the coastline, but also because at a place there could be the land moving up or down, therefore contrasting or accelerating sea level rise [2]. Sea level can change significantly from one coastal location to another, as a result of a number of ocean, atmospheric and land processes that occur at various spatial and temporal scales [3].

In a global change scenario, as speculated in Li et al. [4], a slow rise of the sea level of few cm associated to climate change would make a difference to the coastline. It would not retreat from land, making it permanent. The flooding line of transient events (e.g., storm surges, tsunamis, etc.) would also uplift, increasing the risk of more frequent land inundation and more inland propagation [5]. An example is the City of Venice that has long been vulnerable to short duration flooding during winter [6]. The problem was so important that a system of 78 storm gates, known as MOSE [7] has been constructed to protect the city when high water is expected in Venice [8]. Long-term rising sea levels will represent additional challenges in the future [8].

Understanding the climate-related contribution to the sea level change and how much it will likely affect coastal regions is a major challenge, as it also requires localscale measurements of the land effects. In this chapter, we review the sea level trend measuring system involving the integration of recent satellite-based observations from radar altimeters and Global Positioning System (GPS) receivers with historical data from tide gauge stations. The latest advances in open ocean and coastal altimetry to measure sea level changes close to the coasts over the satellite radar altimetry era are also summarized. A more robust inverse method to estimate sea level trends is also presented. It has been tested in the Northern Adriatic Sea, where GPS data are available to conduct a realistic assessment of uncertainties.
