This thesis in an invistigation into the concept of "harm" and its moral relevance. A common view is that an analysis of harm should include a counterfactual condition: an act harms a person iff it makes that person worse off. A common objection to the moral relevance of harm, thus understood, is the non-identity problem. This thesis criticises the counterfactual condition, argues for an alternative analysis and that harm plays two important normative roles. The main ground for rejecting the counterfactual condition is that it has unacceptable consequences in cases of overdetermination and pre-emption. Several modifications to the condition are considered but all fail to solve this problem. According to the alternative analysis to do harm is to perform an act which (1) is responsible for the obtaining of a state of affairs which (2) makes a person’s life go worse. It is argued that (1) should be understood in terms of counterfactual dependence. This claim is defended against counterexamples based on redundant causation. An analysis of (2) is also provided using the notion of a well-being function. It is argued that by introducing this notion it is possible to analyse contributive value without making use of counterfactual comparisons and to solve the non-identity problem. Regarding the normative importance of harm, a popular intuition is that there is an asymmetry in our obligations to future people: that a person would have a life worth living were she to exist is not a reason in favour of creating that person while that a person would have a life not worth living is a reason against creating that person. It is argued that the asymmetry can be classified as a moral option grounded in autonomy. Central to this defence is the suggestion that harm is relevant to understanding autonomy. Autonomy involves partly the freedom to pursue one’s own aims as long as one does no harm.