Structural frame systems may be analyzed and designed based on forces from conventional first order theory when second order effects of axial forces on displacements in the compression members of the system are negligible. This is often the case. Slenderness limits at which this is the case are studied in this paper. Existing limits are reviewed and major factors that affect such limits are investigated. A new slenderness measure, labeled normalized slenderness, is defined based on a reduced stiffness concept. In addition to the geometrical slenderness, the normalized slenderness is a function of axial force and reinforcement. Slenderness limit predictions are obtained from numerical nonlinear analyses for unbraced (sway) members and for transversely loaded braced members. For such members, new slenderness limits are proposed and verified against the nonlinear results. They are generally found to be more reliable than existing lower limit formulations. Braced compression members subjected to end moments only are dealt with in a separate paper.