Ecological economists aim to study the interrelations between ecological and economic systems, with a view to promoting changes towards sustainability. They agree that the nature of global environmental crises is systemic and rooted in economic dynamics. Yet, core economic categories, such as prices, profits, money and their interrelations with ecological destruction, are surprisingly undertheorized. The ‘economic’ becomes either tied to neoclassical economics or remains neglected. Marxian Political Economy has the potential to address this gap, but is mostly ignored in ecological economics, especially in what it is best at: explaining dynamics of the capitalist system as a whole. This PhD advocates a more systemic and theoretically grounded ecological economics. I find that neoclassical economic reasoning is rooted much deeper in ecological economics than often assumed. I present a critique of the neoclassical underpinnings of ecological economics, and a Marxian alternative. I identify the Marxian understanding of ‘value’ and ‘capital’ as the missing core of ecological economics: a realistic understanding of the capitalist system, spelled out from basics in simple and abstract terms. I explain these foundations and integrate them with a system dynamics understanding of global ecological destruction, social crises and barriers to social change. I conclude that ecological economics needs to be grounded in a realistic understanding of capitalism – if it aspires to meet its own ambitions. Economic theory and methodology are powerful political tools towards this end: they expose or conceal root causes of social ecological problems and offer better or worse guidance on how to act. Pragmatism about the role of theory and methodology is dangerous, more often than not, because it promotes the reproduction of power relations that prevent, instead of encourage, sustainability transitions. I argue for the need to break with existing prejudices against the Marxian approach and take it seriously as realistic economic theory