: Modeling of droplet deformation and breakup in spray formation process is a challenging problem. The breakup process affects spray characteristics in different ways including the distribution of droplets and tip penetration length. Preliminary spray simulations done using OpenFOAM code indicate that the results are highly dependant to the choice of breakup model. Therefore, development of a proper droplet breakup model was selected as the main objective of this study. In this regard, a novel analytical framework is established for modeling temporal evolution of droplet deformation and breakup. The approach is based on virtual work principle and potential flow assumption. It benefits from the features of low computational expenses and minimum need for calibration factors, which make it suitable for use in Lagrangian droplet tracking schemes. Based on this framework, a new droplet deformation and breakup model is proposed, which is capable of keeping track of droplet deformation and distortion up to the onset of the bag rupture in Bag breakup regime whereas other analytical models of this class have failed to do so. This is the first model to formulate dynamics of the bag with a two-degree-of-freedom system. In order to calculate model coefficients, The Laplace equation is solved for more that 500 droplet states with different B.Cs. Outcomes from the presented models are promising. The critical Weber number, in which the drop begins to disintegrate, has been predicted with an error of around 20 percent as compared to the experimental data. Models were enhanced after a minor tuning, which result in the critical Weber number of 12.5. Predicted distortion quantities as well as the drop profiles are validated against experiments for both Vibrational and Bag regime. A good agreement is found between the computed results and experiments. A breakup condition is also proposed for determining the onset of bag rapture. The predictions of tuned models for breakup time are acceptable at near-critical Weber numbers whereas they increasingly underestimate the breakup time for higher Weber numbers. Nevertheless, the predicted breakup times are still far superior to those of ellipsoidal models. Within the Vibrational regime the results are almost similar for all the models and match that of TAB model. However, for low Weber numbers (small amplitudes) the frequency of oscillation becomes higher in the present models. Some other minor contributions have been also carried out during the present study. The first one is an approximation for estimating the local pressure coefficient on the surface of non-spherical drops which is used in the aerodynamic calculations. The second one is the modification of dynamic drag model in order to make it applicable up to the end of bag formation stage. The third is a numerical simulation of droplet breakup using Volume of Fluid (VoF) method which provides physical understanding of actual breaking process as well as validation of the assumption used in the modelling. Overall, achievements of the present work indicate a great potential of current approach for modelling droplet dynamics, and it is expected to expand the research field of multiphase flows modeling and simulation. Keywords: spray formation, drop deformation and dynamics, secondary breakup models