Abstract: Effective Field Theories (EFTs) are widely applied in particle physics of and beyond the Standard Model. They also serve as the foundation to understand renormalizations in Quantum Field Theories. In this talk, I will discuss a few crucial aspects of EFTs, including how to define EFTs, how to apply EFTs, and how to handle field redefinitions in EFTs. Modern technologies involved are the Hilbert series method, functional matching with covariant derivative expansion, and geometric formulation of EFTs. I will explain the key ideas of these technologies, and how they facilitate the phenomenological studies in high energy physics.