Reinforced concrete (RC) flat-plate structural systems are considered as key members in RC buildings, failure of which may result in progressive collapse of the building. Flat slab may fail by punching shear failure, flexural failure, or combined flexural-shear failure. Punching failure is associated with a particular collapse mechanism in which the column together with an attached portion of the slab pushes through the surrounding slab. Flexural failure is a ductile failure through which flexural cracks propagate in the tension face of the slab and large deflections are experienced by the slab. Which of the above failures occurs highly depends on the internal reinforcement of the slab. External strengthening of a slab also affects its failure mode. The present thesis is concerned with study of the flexural behavior of flat slabs, as well as improving the behavior of slabs with low flexural strength. For this purpose, two groups of slab specimens were tested. In the first group, all the specimens were designed with similar flexural reinforcement, but were strengthened with different external strengthening systems. In the second group, the specimens were designed differently and the effect of the internal reinforcement on the slabs’ behavior was studied. The specimens in the first group were designed with flexural reinforcement ratio of 0.22%, representing flat slabs which are weak in flexure. One specimen was considered as the reference specimen and seven specimens were externally strengthened. Flexural strengthening of the specimens was performed with carbon fiber-reinforced polymer (CFRP) sheets installed with externally bonded reinforcement (EBR) and externally bonded reinforcement on grooves (EBROG) methods. In order to improve the slabs’ behavior and to constitute a maximum utilization of the strength and strain capacity of the FRP, steel bolts were used for strengthening of the specimens against punching shear. Different patterns of the FRP sheets were also investigated in this group. Tests showed that in the slab without shear bolts, EBROG technique changed the mode of failure from FRP debonding to punching shear mode. Simultaneous flexural and punching strengthening of slab utilizing EBROG method resulted in 57% increase in the load bearing capacity. Flexural and flexural-punching strengthening of the slab specimens also improved their serviceability by decreasing their ultimate deflection, limiting the width of the flexural cracks and their propagation, increasing the cracking load, and increasing the load level corresponding to serviceability limit state. Flexural and flexural-punching strengthening decreased the ductility of the slab specimens. The second group consists of six thin sla four slabs with flexural reinforcement ratio of 0.38% and two slabs with flexural reinforcement ratio of 1.00%. Three of these specimens were reinforced with shear headed studs as internal shear reinforcement. Results show that decreasing the flexural reinforcement ratio as well as employing shear reinforcement change the behavior of slab to a more ductile manner; however, punching shear failure is an unavoidable phenomenon in slab-column connections. In this group, two specimens were strengthened in flexure with FRP sheets using EBROG technique and the efficiency of external flexural strengthening was compared with that of internal flexural reinforcement. According to the results, strain-hardening of the reinforcement bars considerably affected the flexural behavior of the slabs with no FRP strengthening, while its effect on the behavior of the FRP-strengthened specimens was negligible. In addition to the experimental investigations, analytical studies were also accomplished. In the first part of the analytical studies, load capacity of the experimental specimens was estimated using yield-line theory as well as codes’ expressions. A new approach was also developed in this part to estimate the flexural strength of FRP-strengthened flat slabs by extending the yield-line theory for such slabs. In the second part of the analytical studies, critical shear crack theory (CSCT) was used to predict the behavior and failure mode of the experimental specimens. The analytical results were compatible with the experimental ones. It was also shown that the effect of strain-hardening should be taken into account in the theoretical calculations for the thin slabs in the second group. Key Words RC flat slab, flexural behavior, FRP sheet, EBROG technique, yield-line theory, critical shear crack theory.