What is Mafia is the first English translation of the book Cosa è la Mafia by Gaetano Mosca, an Italian political scientist, journalist and public servant. This book is a transcript of a lecture by Gaetano Mosca in Milan in 1900, when he tried to define the essential characteristics of the phenomenon Mafia. These characteristics are the spirit of Mafia, which permeates Sicilian society, the omertà which build up a perfect environment where the Mafia's activities can easily develop, and the ability of the Mafia's cosche to take advantage of their local power linking it to political and financial power. Nowadays, when we discuss about Mafia, it seems that there is an underestimation of this phenomenon, in addition to political and cultural limits to the approach to this problem. All problems that more than one hundred years ago Mosca already noticed and to which tried to give an answer. For this reason, we believe that the translation and the publication of a book already older than one hundred years, will help us take advantage of a witness who tells us about a world far away in time, but close to our interests. The interests of people who are still struggling with the same criminal phenomenon that afflicted Italy at the beginning of the twentieth century. We want to thank Dr. David O'Kane for his contribute to this book and Miss Dela Buhle Gwala, without whose invaluable assistance this book would never have come to light.