The recent financial crisis has simultaneously underlined the importance of systemic risk and the absence of an appropriate framework for assessing, monitoring and regulating it. Modeling systemic risk requires to change the traditional focus of risk modeling and examine the structure and stability of the financial system as a whole, with special attention given to contagion mechanisms which may lead to large scale instabilities in the financial system. We present some recent work on the quantitative modeling of systemic risk, focusing on three key channels for financial contagion: balance sheet contagion, illiquidity cascades and price-mediated contagion generated by feedback effects. Finally, we discuss the implications of these results for monitoring of systemic risk.