This dissertation is a discussion of the challenge that cases of over-determination pose to Act-Consequentialism. Although there are many realistic examples of such cases – for example, pollution, overfishing, or the election of an inappropriate politician – I consider structurally purer examples, one of which I call “Case One.” Suppose that you and I independently shoot and kill a third person called “Victim.” Our bullets arrive at the same time and each shot would have killed Victim by itself. Finally, Victim would not have been killed, if neither of us had pulled the trigger. According to the Standard Version of Act-Consequentialism, an action is wrong if and only if it has an alternative whose consequences would be intrinsically better. Case One challenges the Standard Version because there does not seem to be such an alternative to my action: Victim would have died by your shot if I had not shot him, and similar remarks apply to your action. The dissertation is structured as follows. After Chapter One, which briefly introduces the main issues of the dissertation, I turn to Chapter Two – “Preliminaries” – where I outline the Standard Version and highlight the main characteristics of over-determination cases. These cases are divided into cases of redundant difference making and cases of redundant causation. Cases of redundant causation are subdivided further into cases of causal over-determination and pre-emption. I make an important stipulation in this chapter. I say that our actions in Case One and similar cases are “redundant negative difference makers.” In Chapter Three – “Replies” – I consider whether the proponent of Act-Consequentialism might question the intuition that you and I, respectively, act wrongly in Case One. The proponent might accept that we have this intuition but explain it away, or she might deny that we have the intuition and instead point to something else that is wrong in this kind of case. For example, she could suggest that although neither you nor I act wrongly individually, we act wrongly together. I argue that these replies are problematic.  For instance, explaining away this intuition might also force us to explain away intuitions that support the Standard Version. In Chapter Four – “Causal Consequences” – I discuss an alternative version of Act-Consequentialism that might seem to fare better. The Standard Version interprets the term “outcome of an action” as referring to the entire possible world that would obtain, if the action were performed. The version I have in mind, the “Causal Consequences Version of Act-Consequentialism,” understands “outcome of an action” as instead referring to the causal consequences of the action. It seems clear that you and I, respectively, cause the state of affairs that Victim dies in Case One. However, I show that the Causal Consequences Version has a number of unattractive implications. In Chapter Five – “the Non-Standard Version” – I suggest another alternative version of Act-Consequentialism. This version – the Non-Standard Version – implies that you and I act wrongly in Case One. Roughly, the Non-Standard Version says that an action is wrong if and only if it has an alternative whose consequences would be intrinsically better, or is a redundant negative difference maker. The Non-Standard Version is similar to a principle suggested by Derek Parfit. However, I shall argue that the Non-Standard Version is preferable to Parfit’s principle. In Chapter Six – “Further Cases” – I discuss a number of cases that challenge the Non-Standard Version. For example, what would the Non-Standard Version imply in a case very similar to Case One but where I would have killed another person, if I had not shot Victim? I argue that the Non-Standard Version handles this and other problematic cases, and that it is therefore a plausible alternative to the Standard Version.