This chapter deals with the postbuckling analysis of shallow shells using secant matrices based on Marguerre’s shell theory. Thin shell structures are common in civil, aerospace, and marine fields and are particularly susceptible to elastic buckling due to their slenderness. The previous chapter dealt with secant matrices for the postbuckling analysis of initially imperfect thin plates. This chapter extends Marguerre’s shell theory into the nonlinear postbuckling domain by incorporating the initial imperfections as the shape of the shallow shell. Hence, we see that the ADF formulation presented in Chap. 5 is accurate enough that it also applies to shallow shells with the shape of the shell modeled as an imperfection from the flat geometry. Several examples of shallow shells under transverse and axial loads are presented in this chapter, validating ADF formulation. Results illustrate the effectiveness of the secant matrix method in capturing the complex postbuckling response of shallow shells.