For multicultural states, federalism may offer effective instruments to accommodate diversity while simultaneously promoting common values and creating a feeling of national unity. Spain and Switzerland both have to deal with pluralism. The strongest forms of cultural, linguistic and – in the case of Spain – national diversities are predominantly regionally concentrated in the Spanish autonomous communities and the Swiss cantons and have strengthened the feelings of identity in these regions. This thesis compares how the Spanish and Swiss state systems address diversity in unity through federal elements and institutions. The constitutional comparison analyses the most important federal characteristics of the two states, including the autonomy of the constituent units, how the constituent units are represented in the institutions of the central government, constitutional-amendment procedures, the institution of a neutral umpire charged with settling conflicts between the two orders of government, and intergovernmental relations among the constituent units.