This chapter deals with the behaviour of slender members subjected to high axial forces. For these members, the deflections due to flexural loads lead to sensible eccentricities of the axial forces that add relevant contributions to the bending moments. These contributions cannot be any more neglected, as done within the first-order theory presented in the previous chapters. So the frame analysis shall move within a second-order theory as presented in what follows. Single members under eccentric tension or compression forces are firstly analysed, moving then to more complex structural assemblies, such as continuous beams or columns and frames, to which the criteria of the Force and Displacements Methods are extended, concluding with the presentation of the special calculation problems of the instability analysis of frame structures.