In this lecture, I will briefly introduce phase transitions in the early Universe for the target audience of graduate students who are familiar with quantum field theory and early Universe cosmology. The lecture will be composed of three parts. In the first part, the thermal effective potential will be introduced and I will explain its physical meaning and how we calculate it. Then I will explain phase transitions in the early Universe, especially focusing on first-order phase transitions. The last part of the talk is about their applications in BSM physics and discussion.