This article reviews the history and the latest developments in the field of Business and Human Rights, considered within the framework of Public International Law. The study departs from an initial stage when the issue was governed by principles and recommendations pertaining to the sphere of Corporate Social Responsibility (CSR), which was to be qualified in any case as soft law. Today, we are moving towards a hardening of the system that is leading to a considerably hard law Corporate Responsibility for Human Rights (CRHR) in Public International Law. This process is currently underway through several complementary and simultaneous channels, of very diverse and not always precise legal nature, which the author reviews: (a) the development of the Guiding Principles by the UN; (b) the search for a controversial new international treaty; (c) the work of the UN Treaty Bodies on the subject; (d) the role of the OHCHR; (e) the role of the OECD; (f) the national human rights plans; and (g) the interplay of multiple jurisdictions. The author proposes that this complex, flexible, sometimes ambiguous and changing interaction of elements of a very different legal nature offers as many opportunities for the development of the cause of human rights as it, at times, risks the loss of legal certainty to the detriment of the actors operating in the system. He argues that in the highly desirable progressive development of Public International Law in this area, a balance must be struck that protects the legitimate claims of all parties to legal certainty and to the rule of law, if its important achievements are to be protected and consolidated.