In this thesis punching shear behavior of reinforced concrete slabs is studied in detail. The study is mainly organized in two parts. The first part mainly focuses on provision of simple, safe, economical and realistic design equation formulation for the design of slabs against punching shear. The second part proposes an innovative strengthening approach for existing RC slabs to enhance their capacity against punching shear. For this, a low-invasive, efficient and effective system of post-installed reinforcement as punching shear reinforcement is utilized. Reinforced concrete slabs resting directly on column are popular slab systems for reinforced concrete structures. Their usage is quite common in industrial structures (typically with post-tensioning) and in buildings used for parking purposes, since they allow higher head room for the same story height compared to traditional beam-slab systems. However, these slabs are susceptible against punching failure at column locations and catastrophic failures of slabs have been reported in the past due to punching. This highlights a lack of understanding and calls for a rational approach which can effectively describe and quantify the mechanism of punching shear. Almost all codes propose empirical equations based on statistical evaluation of existing experimental data to estimate the punching shear capacity of the reinforced concrete slabs. In the first part of this thesis, through an extensive numerical parametric study a new, simple, and realistic punching shear predictive equation is proposed. The numerical study is carried out using the software MASA developed at the University of Stuttgart, which is known for its capabilities to simulate the nonlinear concrete behavior quite well. The numerical modeling approach is first verified against existing experimental results and then used to carry out a detailed parametric study. The proposed equation considers the quantitative contribution of concrete grade, flexural reinforcement ratio, member depth and loading column size. The prediction capacity of the proposed equation is tested by comparing it with existing 235 test data bank. It has been shown that the proposed equation is able to predict the punching shear capacity of the RC slabs with high accuracy and significantly better than the current formulations given in different codes and standards. The second part of the thesis is invested on studying the strengthening of existing reinforced concrete slabs by introducing post installed reinforcing bars as punching shear reinforcement, through numerical and experimental study. The novelty lies in the fact that for the first time the effectiveness and efficiency of plan and vertical arrangement of post installed shear reinforcement is evaluated in this work. A well-designed experimental program that evaluates the effect of arrangement of post installed shear reinforcement is developed and executed. It is shown that the punching shear strength depends not only on the amount of introduced shear reinforcement but also on their plan and vertical arrangement. Furthermore, the effect of member depth on the effectiveness of post installed shear reinforcement is analyzed and it is shown that the percentage utilization of the reinforcement is significantly affected by the member depth. Eventually, an empirical equation that predicts the punching shear capacity of slab strengthened with post installed shear reinforcement is proposed. The degree of accuracy and the reliability of the proposed equation is verified using experimentally tested slabs in this thesis, previous experiment as well as numerically tested slabs, there is a good agreement between the calculated value and experimentally and numerically tested slabs.