The information systems (IS) field has relied on a broad and varied collection of theories. The core of this theoretical landscape is an important determinant of the identity of the IS field, and has hence been discussed extensively in recent years. Nevertheless, little is known regarding the composition, consistency, and evolution of this theoretical core. Here we address these issues empirically by identifying the theories and levels of analysis which constitute the core of the IS field, and by quantifying the degree of theoretical uniformity among the field's top journals. Using a set of 87 theories in conjunction with n-gram analyses, we analyze the full text of every research article published in three top IS journals over a 22-year period in order to identify the field's most frequently and consistently used theories and theory clusters. We further identify substantial overlap among the theoretical core of the field's leading journals, but also note differences in the theoretical character of each journal.