The strength of structures undergoing bifurcation is affected by imperfections. The mechanism of such a dependence of the strength has been conventionally described by “imperfection sensitivity laws.” We follow the Liapunov–Schmidt–Koiter approach and introduce imperfection sensitivity laws for simple critical points of various kinds that are described in Chap. 2, which is a prerequisite of this chapter. This chapter lays a foundation of Chaps. 4 – 6 and is extended to a system with group symmetry in Chap. 9.