3.1 Introduction Surveying for bats and their roosts is an important aspect of bat work for both research and conservation purposes. Until recently, bats were probably one of the most under-recorded groups of vertebrates. However, rising interest and the accumulation of data from enquiries and other initiatives have led to a great increase in the number of records and a better idea of the distribution and relative abundance of the various species. Advances in equipment, notably bat detectors, have increased the potential for surveying bats away from roosts and created a doorway to understanding the foraging and habitat needs of bats. A National Bat Monitoring Programme was initiated in 1996 and is run by the Bat Conservation Trust. This aims to track the population changes of target species on a UK-wide basis. The Agreement on the Conservation of Populations of European Bats also has target species for Europe-wide monitoring. For the purpose of this chapter, the monitoring of sites and bat populations can be considered as an extension of surveying. Survey includes the discovery of bat sites and the mapping of species distribution, key bat roosting / hibernation sites and feeding areas. Monitoring is the repeated counting of bats either in roosting / hibernation sites or feeding areas over a period of time. Specialised techniques such as ringing and other types of marking are covered in later chapters.