This volume opens up stimulating new perspectives on a broad variety of Barcan Marcus’s concerns ranging from the systematic foundation and interpretation of quantified modal logic, nature of extensionality, necessity of identity, direct reference theory for proper names, notions of essentialism, second-order modal logic, modal metaphysics, properties and classes, substitutional and objectual quantification, actualism, the Barcan formula, possibilia and possible-world semantics to epistemic and deontic modalities, non-language-centered theories of belief, accounts of rationality, consistency of a moral code, moral dilemmas, and much more. The contributions demonstrate that Barcan Marcus’s original and clear ideas have had a formative influence on the direction in which certain themes central to today’s philosophical debate have developed. Furthermore, the volume includes an illuminating intellectual autobiography from Barcan Marcus herself as well as an informal interview containing her unfiltered, frank answers. The book brings together contributions by Ruth Barcan Marcus, Timothy Williamson, Dagfinn Føllesdal, Joëlle Proust, Pascal Engel, Edgar Morscher, Erik J. Olsson, and Michael Frauchiger.