Effects of axial forces on the displacements in compression members, generally termed second order effects, are small and can be neglected in a great many structures. In recognition of this, most design codes and standards for reinforced concrete structures give lower slenderness limits for compression members, and allow structures to be designed for forces obtained by conventional first order theory when these limits are not exceeded. The paper studies such limits for unbraced (sway) members and for transversely loaded braced members. Existing limits are reviewed, major factors that affect lower limits are investigated and slenderness limit predictions are obtained from numerical nonlinear analyses. Finally, a new lower slenderness limit formulation is presented. The slenderness is defined in terms of a so-called normalised slenderness. In addition to the geometrical slenderness, it is a function of axial force and reinforcement. The formulation is rational and may replace, or used as an alternative to, existing lower limit formulations.