Although an exact definition of a deep beam in structural engineering is yet to be agreed on universally, it is widely accepted that a structural deep beam action occurs at span/depth ratios less than about 2.5. These structural elements, with depths greater than normal, do not obey the Bernoulli hypothesis stipulating that plane section before bending remains plane after bending. Consequently there may be more than one neutral axis at a given section of a deep beam. This difference in behaviour under load compared to that of normal beams requires special considerations in analysis, design and reinforcement detailing. The development of distinctive diagonal cracking from support to loading regions is a feature of deep beam behaviour. After the development of this diagonal cracking, a deep beam takes up further loading through an arch action, exhibiting greater strength reserve compared to an ordinary beam for which the ultimate load is sensibly the same as the diagonal cracking load. In slender deep beams, the interaction between in-plane shear and out-of-plane bending caused by slenderness effect influences the nature of this diagonal cracking and hence, affects their ultimate behaviour. This slight difference in behaviour under load between stocky deep beams and slender deep beams is clearly illustrated in the specimens tested to destruction by the author and explained by a Mohr circle analysis in the present work. This paper examines the ultimate behaviour of concrete deep beams through the literatures and through an extensive on-going experimental research work carried out by the author. It particularly stresses on the behaviour of slender deep beams under shear. In effect, with the advances of the materials’ technology and the increasing improvement of the concrete strengths combined with the search for an economical design, concrete sections are becoming slender and consequently the slenderness effect can no more be neglected as in stocky deep beams, even when designing for shear.