The link between William Booth and Karl Marx is not simply that they were in London at the same time and were both interested in and affected by the unemployed population there. There was a certain homomorphism in the way that the London residuum impacted upon them. Friedrich Engels wrote in Dialectics of Nature about the law of the transformation of quantity into quality, and vice versa. The 'nodal points' in quantity that had a qualitative impact and influenced Marx's thought were the size of the residuum in London and in particular the size of the reserve army of labour. The quantitative features of the East End that influenced Booth's were the size of the residuum and the length of time the existence of the submerged tenth remained unchallenged. This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book.