This book addresses the principle of equality of arms and its impact on the Brazilian criminal procedure. It begins with the connection understood to exist between this principle and the constitutional matrix of equal treatment in the process. Starting from this constitutional framework, the focus is on equality as a fundamental rights norm, leading to the core issue: equality of arms as an emanation of equality and its impact on criminal proceedings. A brief overview of some of the main characteristics of contemporary criminal procedural systems is then necessary. This is done either to support the understanding that no single system truly exists or to position equality of arms closer to the evolution of the so-called adversarial system. The analysis outlines the concept of equality of arms based on comparative criminal procedural law, notably drawing from the understandings of international bodies that pay detailed attention to it, such as the European Court of Human Rights and the United Nations Human Rights Committee. By considering foreign criminal procedural law doctrine, or its development outside Brazil, the aim is to test the application hypotheses in the Brazilian criminal procedural scenario. The work particularly examines the incidence of equality of arms in various aspects of current and projected Brazilian criminal procedural law. Ultimately, recognizing the principle's incidence and its character as a fundamental normative principle based on the constitutional backdrop, propositions are formulated to better delineate Brazilian criminal procedural law in adherence to this principle.