In this paper I describe in detail the concepts of volatility, diversification and contagion, three basic keys to understand the seemingly whimsical behaviour of financial markets. The presentation is deliberately non-technical and largely self-contained, with most required concepts defined along the way. Nevertheless, the analysis is mostly empirically oriented, with an emphasis on the methods that have been proposed to measure those concepts and a discussion of the stylised facts that the resulting measures imply. I also use those measures to study the effects of the financial crisis of 2007-2008 and the euro sovereign debt crisis of 2010-2012 on Spain.

Key words: correlation, dependence, stock markets, volatility derivatives.
JEL classification: C58, G11, G32.