The book assesses the principles of international investment law in contemporary international investment rule-making and the underlying arbitral practice. While delving into the fragmentation in international investment rule-making and the attendant legitimacy challenges, the book proffers options for normative coherence. The analysis transcends the principles and practice of international investment law and contextualises competing norms, namely: environmental sustainability, labour rights, business and human rights, corporate social responsibility and sustainable development. The book targets students, instructors, practitioners and policy makers in the fields of International Investment Law, International Economic Law, Oil and Gas Law, Petroleum Law and Energy and Natural Resourc¬es Law. The extensive coverage of the regulatory frameworks of inter¬national investment from national, regional and international perspec¬tives makes the book a vital material for teaching in every university. The book presents complementarities between the principles and prac¬tice and offers a roadmap for the future.