Freshwater resources have been impacted both quantitatively and qualitatively because of rising human demand and increasing pollution. In particular, lakes and reservoirs are not only a major source of renewable fresh water for human societies in many parts of the world; they provide society with many other benefits. Concomitantly with increasing water demand and pollution, climate change and variability are expected to directly affect the physics, chemistry and biology of these systems. It is difficult to separate the effects of climate change from other human impacts on lakes and these may work synergistically to increase effects. In addition, there are few long-term studies of lakes and reservoirs, especially of their biological properties, to provide the data necessary to assess potential climate change effects. To date the results from different parts of the world are mixed, with some analyses showing measurable effects and others none. The UNEP GEMS/Water Programme is a programme that compiles water physical-chemical data for lakes and reservoirs around the world. The GEMS/Water database, GLOWDAT, is potentially an important tool to use in making climate change and variability impact assessments of water quality since records go back for many systems to 1978. It is continuously updated and its geographical coverage and number of surface water systems for which records are available are increasing. However, it contains limited biological data and most data is for surface samples that are highly influenced by local, short term changes. Focused, issue-driven long-term monitoring programmes are essential for assessing the impacts of climate change and variability, as well as other anthropogenic impacts, on lakes and reservoirs.