A polymer is a very large molecule consisting of many atoms covalently bonded like a chain. Polymers take a random coil conformation in solution and entangle each other when the polymer concentration is high. The unique structure gives unique physical properties to polymer solutions. This book is an introduction to the modern theory of polymer physics. It describes basic concepts and methods to discuss the statistical properties of the assembly of chain-like molecules. This involves scaling theory, concentration fluctuation, gels and reptation.