The subject of this book is the action of unconstitutionality, which is the most important defense mechanism of the Political Constitution in Colombia. The presentation has been arranged in five sections. The first one relates to the institutional assumptions of the action, such as the Constitution, the principle of supremacy, the control of constitutionality and the Constitutional Court. The remaining four sections develop each of the related procedural institutions, such as the action, the object in the action of unconstitutionality, the constitutional process and the constitutional sentence. This work is a contribution to the task of consolidating constitutional procedural law in Colombia.