This chapter provides an overview of unified theories of elementary particles and the inflationary universe scenario. It begins by discussing the role of scalar fields in these theories, including spontaneous symmetry breaking and the Higgs mechanism. The chapter then explores phase transitions in gauge theories, both at high temperatures and in cold superdense matter, and the implications for the early universe. The behavior of the effective potential at finite temperatures is analyzed, leading to the concept of a second-order phase transition. The chapter also covers the hot universe theory, including the Friedmann equations and the evolution of the scale factor, and the implications of these theories for the early universe, such as the Big Bang and the Big Crunch. The discovery of the microwave background radiation in 1964-65 is highlighted as a critical event that confirmed the hot universe theory.This chapter provides an overview of unified theories of elementary particles and the inflationary universe scenario. It begins by discussing the role of scalar fields in these theories, including spontaneous symmetry breaking and the Higgs mechanism. The chapter then explores phase transitions in gauge theories, both at high temperatures and in cold superdense matter, and the implications for the early universe. The behavior of the effective potential at finite temperatures is analyzed, leading to the concept of a second-order phase transition. The chapter also covers the hot universe theory, including the Friedmann equations and the evolution of the scale factor, and the implications of these theories for the early universe, such as the Big Bang and the Big Crunch. The discovery of the microwave background radiation in 1964-65 is highlighted as a critical event that confirmed the hot universe theory.