The 20th century was known as the century of physics. In the past 120 years, concepts such as space, time, energy, entropy and particles were understood to much deeper levels. New paradigms of thinking such as relativity and quantum mechanics emerged. This course is the third course in the Understanding Modern Physics series, which covers an introduction to action principle, fundamental particle physics, entropy & information and complexity. This course offers a broad view between simplicity and complexity, to complement the first and second course in the Understanding Modern Physics series.