The statistical methods have been developed to help in the quantitative study of variation and the analysis and interpretation of data from experiments that are subject to variation. In biology, the amount of variation and the relative importance of the different causes of variation are often of interest in themselves. The chapter explores a few situations in which variation occurs, sometimes without people noticing it. These examples come from very different situations and the forms of the variation are particular to the situation. The amount of variation in biological material is generally much greater than in inanimate material. The greater variability of living material emphasizes the particular need for methods for dealing with variation in the biological sciences, medicine, and agriculture. The objective of agricultural experimenters is generally to measure the effect of varying some factor, such as, for example, the level of protein in poultry diets.