The Aquatic Warbler Memorandum of Understanding (MoU) was concluded in Minsk, Belarus, under the auspices of the Convention on Migratory Species (CMS) and became effective on 30 April 2003. It aims to safeguard the Aquatic Warbler Acrocephalus paludicola that is the rarest migratory songbird of Europe, and the only globally threatened passerine bird found in mainland Europe.

Once widespread and numerous in fen mires and wet meadows throughout Europe, the Aquatic Warbler has disappeared from most of its former range. Nowadays, its world population of only 9,000-14,000 vocalising males is confined to fewer than 40 regularly occupied breeding sites in only five countries, covering together only less than 400 km2 (area of occupancy) with four sites supporting over 80% of the global population.