This thesis is about whether it is a good idea to place monetary value on nature, to remedy the fact that we treat it as having no particular value to us humans, although it clearly has. The thesis is based on five research papers that can be said to position themselves on opposite sides in the debate on monetisation of nature. The first two papers consider the basis of neoclassical environmental economics and apply the value theory and valuation methods from normative neoclassical welfare theory, on which monetisation of nature is based. The other three papers examine, with increasing degrees of criticism, this theory of value and especially its central assumption that value can be derived from people’s choices, or “revealed preferences”. The thesis itself is a “reflective story” about the journey I made as I learned to think about and understand neoclassical environmental economics in new ways. I reflect upon my work from a philosophy of science perspective, consider how for-granted-taken ideas from neoclassical economics affect environmental economic analysis and its conclusions, and examine the subject of value and valuation from what has become my new theoretical standpoint, ecological economics. It concerns meta-theoretical questions about ontology, that is, ideas in a research discipline about how things really are (what is), and epistemology, ideas about how researchers can provide relevant knowledge about reality. Such ideas are often taken for granted in neoclassical economic analysis and how they affect the analysis and its conclusions is not seldom unreflective. In the thesis, I move from explaining why neoclassical environmental economists advocate monetisation and pricing of nature as important solutions to environmental problems, to exemplifying how this turned out in research projects intended to serve as decision support in practice, and then to exploring and clarifying an alternative theory of value and valuation from ecological economics based on value pluralism and so-called deliberative ...