Now is an opportune time to be writing about socialism. 2017 and 2018 are landmark years for socialism. 2018, of course, is the 200 year anniversary of the birth of Karl Marx, whose name is mostly associated with modern socialism. 2017 is the 100 year anniversary of the Soviet Revolution which brought into being the first socialist experiment in the world. Furthermore, 2017 is the 150 year anniversary of the publication of Volume One of Marx’s monumental Capital. Finally, 2017 is the 100 year anniversary of the publication of the iconic short booklet, Imperialism, authored by leader of the Soviet revolution, V. I. Lenin. The procedure adopted for this book on socialism in the 21st century is as follows: Chapter One sets the stage for the book by looking at the way the notion of socialism has gained renewed respectability through calls for it to commence via mainstream political parties. Chapter Two considers questions of the sources for socialist ideas and follows how these were progressively shaped to animate revolutionary actors in putting socialist experiments into practice. Chapter Three reviews the record of the Soviet Union’s socialist journey. It examines Soviet discontents and explanations for this movement; it then ends with a review of the initial theoretical response to Soviet failings. Chapter Four explores the transformations undergone by capitalism in the 20th and 21st century. The purpose of this is to show that the kind of capitalist economy socialist revolutionaries believed would be the antechamber for socialism now no longer exists.