Explore the fundamentals of Chemical Reaction Networks in this comprehensive lecture by Supriya Krishnamurthy from the International Centre for Theoretical Sciences. Delve into the world of stochastic chemical reaction networks, examining key examples and definitions. Learn about the concept of deficiency and its significance in chemical reactions. Discover the Deficiency Zero Theorem and its implications for understanding complex chemical systems. This lecture, part of the Bangalore School on Statistical Physics XIII, offers a deep dive into the statistical physics of chemical reactions, bridging the gap between master's-level courses and cutting-edge research topics.