At present there are twenty-five functioning federations worldwide, which contain over forty percent of the world's population. A distinctive feature of federalism is that it has taken a variety of forms, including new variants and innovations. In Comparing Federal Systems Watts provides a clear analysis of the design and operation of a wide range of federations.

Fully updated, this third edition encompasses reference to a wider range of federations and federal experiments. Included are mature federations such as Switzerland, Canada, Austria, Germany, and India; emergent federations such as Mexico, Malaysia, Pakistan, Spain, Brazil, Belgium, Russia, Argentina, Ethiopia, South Africa, Nigeria, and Venezuela; micro-federations such as Micronesia, Belau, St. Kitts and Nevis, and Comoros; federal-confederal hybrids such as the United Arab Emirates and the European Union; and post-conflict federal experiments such as Bosnia and Herzegovina, Sudan, Iraq, and Congo. Federations that have failed are also discussed.

Watts looks at interactions between social diversity and political institutions, the distribution of powers and finances, processes contributing to flexibility or rigidity in adjustment, the extent of internal symmetry or asymmetry, the character of representation in federal institutions, the role of constitutions and courts, provisions for constitutional rights and succession, the degree of centralization and non-centralization, and the pathology of federations."