The human brain is the most complex structure in the universe and yet there are a lot of mysteries about it. There are around one billion people worldwide who suffer from kinds of brain disease, any malfunction of brain activity. Brain stimulations, which are generally divided into invasive and noninvasive methods, are important tools for understanding the brain functions and treating its disorders. Despite many applications in diagnosing and treating brain disorders and improving its functions, conventional noninvasive methods , cannot directly modulate deep brain structures. Direct stimulation of these structures requires invasive methods such as deep brain stimulation, which was invented in the late 1980s. Although invasive methods have been very successful, its applications are very limited due to its significant risks. Noninvasive deep brain stimulation, inspirred by the interferential current therapy, is a bridge between conventional noninvasive methods and deep brain stimulation. It is introduced in 2017 to directly stimulate deep brain structures through scalp electrodes. In this study, the interferential electric fields in a homogeneous and inhomogeneous cylinder and sphere were computed both analytically and numerically. In addition, they were computed numerically in a precise anatomical human head model. Threshold approach, exploiting the maximum envelope modulation amplitude along any orientation, has been used for determining the volume of tissue activated. Results showed that it is possible to directly stimulate deep structures. Also, by extracting the relationship between activated area and stimulation parameters, a method was developed to determine the stimulation parameters in order to achieve the desired activated area (inverse problem). In addition, the activated area was computed more accurately with the help of microscopic axon models. The results can be used to prove this method from the computational point of view. In the following, after designing and implementing the interferential current stimulator, the noninvasive deep brain stimulation method was evaluated in the phantom to validate the concept of the theory presented. In according with the great application and importance of this method and its challenges, more exact investigation and in-vivo evaluation are proposed for future researches.