In the present book, attempts have been made to conquer the difficulty of solving nonlinear differential equations, especially the highly nonlinear ones. A convenient approach (AGM = Akbari-Ganjis method) has been proposed to solve all the existing nonlinear ordinary differential equations up to now. Here, all the existing nonlinear ODEs have been divided into some categories, and for each of them, an innovative technique has been introduced to find their exact solution. Moreover, a suitable technique has been proposed to evaluate the precision of the acquired solution, which can be utilized when there is not any exact solution and the problem is not solvable by numerical methods, such as some kinds of inverse problems. One of the significant nobilities of this book refers to the ability of AGM in solving partial differential equations in different aspectsfor instance, fluid mechanics, heat transfer, and vibration, as discussed in the sixth chapter. Eventually, we hope this book can be considered as a suitable guide for all the people who deal with nonlinear differential equations.