The concept of Riemann surfaces was introduced in Riemann's thesis, and the moduli space of Riemann surfaces was defined by Riemann in a masterpiece a few years later. Due to a broad connection with many subjects in mathematics and physics, Riemann surfaces and their moduli spaces have been intensively studied and should continue to attract attention in years to come. Recently, there has been an explosion of interest in and work on tropical algebraic curves-analogues of algebraic curves over the complex numbers and hence of Riemann surfaces. This book is an accessible introduction to all these topics, with special emphasis given to their many connections with subjects such as algebraic geometry, complex analysis, hyperbolic geometry, topology, geometric group theory, and mathematical physics.