analogy with locally symmetric spaces， which by definition are obtained as orbit space of symmetric spaces endowed with an action of an arithmetic group. This analogy， though imperfect， has been very fruitful in the past decades， as it suggested both problems and solutions.

　　As we mentioned， compact Riemann surfaces can also be regarded as algebraic curves over the complex numbers. In the past few years， there has been an explosion of work on their tropical counterparts， i.e.， algebraic curves over the tropical semifield. These have appeared in many unexpected topics and make currently a very active area of study. It turns out that the moduli spaces of tropical curves also fit into the above analogy，in the sense that these are the orbit spaces of tropical Teichmuller spaces by outer automorphism groups of free groups. The outer automorphism groups of free groups are among the most important groups in geometric group theory， and the tropical Teichmuller spaces were studied as spaces of marked metric graphs before the subject of tropical geometry （and its name） existed.

　　Given the rich history of Riemann surfaces and their moduli spaces， and their generalizations， applications and connections with other subjects， it seems helpful to provide an accessible and timely introduction to all the above topics， while emphasizing the underlying connections. The current book is an attempt towards this goal. It consists of two parts. The first part deals with mapping class groups of surfaces， Teichmuller spaces and their applications to moduli spaces of Riemann surfaces， whereas the second part deals with tropical analogues and some applications in geometric group theory. Though these parts were conceived independently， together they cover both basic and essential results in these subjects as well as some recent developments. Although there exist several works on some of the above topics， we hope and believe that the nature of the subject justifies our offering of our own perspective on this wonderful world.