Behaviour of slender reinforced concrete (RC) beams is different from that of normally propotioned beams, on account of slenderness effects which introduce susceptibility to lateral torsional buckling. Highly slender beams are prone to sudden instability mode of failure. Moderately slender beams are also susceptible to slenderness effects and they may undergo flexural failure at moment values less than their flexural capacities corresponding to material failure (Muf). Concrete design codes presently do not provide any procedure to evaluate the critical buckling moment (Mcr), and also do not account for the capacity reduction in moderately slender beams on account of slenderness. The existing recommendations are limited to prescriptions of limiting slenderness ratios, which are semi-empirical in nature. The experimental results carried out in the present study clearly show reduction in moment capacity in beams with slenderness ratios well below the critical values given in various codes and in literature. Based on the experimental and theoretical studies conducted, new design guidelines are proposed for slender rectangular RC beams.