In the previous chapter we discussed the key ideas involved in factorial experiments and in particular the notions of interaction and of the possibility of extracting useful information from fractions of the full factorial system. We begin the present more specialized chapter with a discussion of confounding, mathematically closely connected with fractional replication but conceptually quite different. We continue with various more specialized topics related to factorial designs, including factors at more than two levels, orthogonal arrays, split unit designs, and response surface methods.