For formal quantitative analysis of PDEs, many approaches are known. A number of averaging techniques are considered tin this chapter that can be validated mathematically. Under certain conditions, one can average an equation where time dependence and a linear operator are present. A major problem is that the “unperturbed” problem can sometimes be understood qualitatively but is difficult to be used to formulate slowly varying variational equations; however, this is possible in a number of cases. Another approach, suitable for boundary value problems, is using projection methods. This will be illustrated in Chap. 14 .