Tide gauge records and satellite altimetry have demonstrated that the sea level is rising on global and relative (regional/local) scales. Globally, the rate of sea-level rise (SLR) in the past two decades is faster than at any time. During the most recent era, 2006–2018, the global SLR rate was 3.7 mm/year, i.e. nearly three times faster than during 1901–1971 (1.3 mm/year). This is mainly attributed to two main reasons: (1) seawater thermal expansion due to climate change and global warming, and (2) ice melting of the Arctic and Antarctic regions. Additionally, the vertical land movement (subsidence/rise) can impact the calculated relative SLR rates. SLR is projected to continue if global warming will continue. SLR has a destructive impact on coastal cities, especially coastal low-lying areas. Factually, it is not only human infrastructures that are at risk from the SLR and coastal flooding, but also coastal environments such as coastal wetlands, seagrass beds, rocky shores, and sandy beaches are vulnerable to such a rise and flooding. This chapter aims at highlighting the SLR issue on global and relative scales, by using both tide gauges and altimeter tools.