This book focuses on the problem of responsibility voids: these are cases where responsibility for a morally undesirable outcome cannot be attributed to any of the involved agents. Responsibility voids are thought to occur in collective decision-making and in the context of artificial intelligent systems. In these cases, philosophers worry that there is a shortfall of moral responsibility. In particular, such voids are often assumed to justify a notion of collective responsibility that cannot be reduced to individual responsibility. One of the aims of the book is to study how collective responsibility and joint action relate to individual responsibility and individual actions. The book offers a unifying framework for modelling moral responsibility by drawing from modal logic and game theory.The book investigates the possibility and scope of the problem of responsibility voids. One of its characteristics is its pluralistic perspective on moral responsibility: in contrast to giving a unique and all-encompassing definition of it, the book makes progress by spelling out and modelling several conceptions of moral responsibility. One of the appealing features of the book is that a relatively small range of models is used to investigate a variety of conceptions of moral responsibility. The unifying framework can thus be used to characterize the conditions under which responsibility voids are ruled out.