Explore the foundations of Effective Field Theories (EFTs) in this introductory lecture by Aneesh Manohar, part of the School for Advanced Topics in Particle Physics (SATPP) at the International Centre for Theoretical Sciences. Delve into the systematic approach of describing physics at different scales using relevant degrees of freedom. Gain insights into how EFTs have become essential tools for understanding the universe, from TeV-scale physics to meson and baryon observables. Learn about the importance of EFTs in modern particle physics and their applications in both low and high energy regimes. Benefit from this comprehensive introduction designed for postdoctoral fellows and advanced PhD students in theoretical particle physics, providing both conceptual understanding and technical tools for critical analysis.