Many ecological studies on animal populations require an estimate of the population size or density of individuals. Distance sampling (DS) is the most commonly used method today. Distance sampling can also be used with indirect counting methods. Another group of methods for determining population size are mark-recapture methods. For these, the animals must be individually recognizable or marked. We discuss how to mark animals of different taxa so that hundreds of individuals can be identified using an adapted numbering key. Many animal species are individually identifiable by facial or color patterns. Photo databases help to archive these patterns.