This paper presents analytical and numerical models for predicting buckle propagation pressure of pipe-in-pipe (PIP) systems. The 2D analytical solution is based on flexural and membrane deformations of the cross section of the PIP system under external pressure. The numerical model uses 3D finite element analysis which is verified against hyperbaric chamber results for a particular PIP system. Results show that the proposed analytical equation provides a lower bound for propagation pressure of PIPs. A parametric study is performed using the FE analysis that shows the significant factors affecting the propagation pressure of PIP systems. The FE models provide valuable information about the buckling modes and progress of the buckle in the carrier and inner pipes.