Interdisciplinarity is considered the best way to face practical research topics since synergy between traditional disciplines has proved very fruitful. Studies on interdisciplinarity from all possible perspectives are increasingly demanded. Different interdisciplinarity measures have been used in case studies but, up to now, no general interdisciplinarity indicator useful for Science Policy purposes has been accepted. The bibliometric methodology presented here provides a general overview of all scientific disciplines, with special attention to their interrelation. This work aims to establish a tentative typology of disciplines and research areas according to their degree of interdisciplinarity. Interdisciplinarity is measured through a series of indicators based on Institute for Scientific Information (ISI) multi-assignation of journals in subject categories. Research areas and categories are described according to the quantity of their links (number of related categories) and their quality (with close or distant categories, diversity, and strength of links). High levels of interrelations between categories are observed. Four different types of categories are found through cluster analysis. This differentiates “big” interdisciplinarity, which links distant categories, from “small” interdisciplinarity, in which close categories are related. The location of specific categories in the clusters is discussed.