Due to the recent raise of oil price, giant oil and gas companies have expanded their exploration to more than 3 kilometres under see level. In extreme conditions of deep under water excavation, pipe-in-pipe systems have become favourable option due to their unique structural capacity. Subsea pipelines and PIP systems experience large bending moments during installation and operation. However, unlike single-walled pipelines, the behaviour of PIPs under bending has been only marginally addressed. In this project, the bending response of the PIP systems with diameter-to-thickness ratio (D/t) of 15 to 40 is investigated. To this purpose Linear bifurcation analyses (LBA) and geometrically nonlinear analyses (GNA) are conducted on PIPs of varying lengths. For the first time, closed formed analytical solution are derived and provided to predict the classical and nonlinear limit moments of PIPs, and are compared to the existing expressions for single-walled pipelines. Ultimate bending moments of PIPs are obtained from physical four-point bending tests and are compared against geometrically and materially nonlinear analyses (GMNA). The finite element results show that in PIPs with centralizers, the limit moments (GNA) drop slightly, however, the ultimate moments (GMNA) remain unchanged. A parametric study of the effect of geometry and material properties of the inner and outer pipes on the ultimate moment of PIPs is presented. It is understood that the ultimate moments of PIPs with thick tubes are predominantly influenced by the material nonlinearities rather than ovalization of the tubes. Moreover, to investigate effect of external pressure on the bending moment capacity of PIPs, Finite Element study has been performed on a PIP system subject to pure bending, pure external pressure and combined actions of external pressure and bending. Results show that bending moment capacity of the PIP systems are more susceptible to the external pressure compared to that of the same single outer pipes.