This dissertation in part aims to use the case of addiction to consider which theory of moral responsibility best accounts for and explains the features core to assessments of blameworthiness. In this project, I argue that Reasons Responsiveness theories provide a better framework for tracking and explaining the features necessary to responsible agency than Quality of Will theories. I also show how the condition of addiction can help refine theoretical commitments made by Reasons Responsiveness theories with regard to assessing blameworthiness for wrongdoing. Further, I give direction as to how we should think about addiction in theorizing about responsibility, including how we might conceptualize the condition, which of its features are most important in assessments of responsibility, and whether, why, and how significantly the condition might undermine responsible agency. After clarifying my assumptions about moral responsibility in chapter 1, in chapter 2 I argue against conceptualizing addiction as irresistibly compelling addictive substance use. I also offer some paradigmatic cases of addiction that have been influential in philosophical discussions about moral responsibility and introduce complications to these cases given the richer notion of addiction I defend. In chapter 3, I challenge the claim that Quality of Will views alone rightly account for moral responsibility in the case of Frankfurt’s so-called “willing addict.” I contend that the stipulations about addiction, wrongdoing, and control in the cases invite implicit assumptions that impact how the cases are often sorted in terms of blameworthiness and excuse. In chapter 4, I contend that structural Quality of Will views depend on the feature of effort to distinguish relevantly different cases of so-called “unwillingness” with regard to wrongdoing. Yet, I argue that this dependence on effort puts the views at a loss for explaining cases in which effort is absent because responsible agency seems undermined in some important way. In chapter 5, I argue that non-structural Quality of Will views cannot explain how addiction can mitigate blameworthiness for addiction-explained wrongdoing. Finally, in chapter 6, I offer guidance for how capacitarian Reasons Responsiveness views might explain the relevance of culpability for addiction in assessments of blameworthiness for addiction-explained wrongdoing.