In this talk, I will explore the generalization of the Feynman path integral in quantum field theory to complexified fields. The main goal is to study the dynamical as well as finite temperature and density properties of quantum field theories starting from first principles, which are often not accessible using standard computational techniques. The approach is based on a mathematical framework known as the Picard-Lefschetz theory, which has profound implications for quantum field theory and string theory. After introducing the basic concepts that link the Feynman path integral to the Picard-Lefcshetz theory, I will explain how this framework can be utilized to achieve the aforementioned goal, opening up a new way of solving previously unsolvable problems with a broad range of applications, ranging from high energy physics to condensed matter physics.