Objective: To develop and compare methods for identifying natural alignments between ambulatory surgery centers (ASCs) and hospitals that anchor local health systems. Measures: Using all-payer data from Florida's State Ambulatory Surgery and Inpatient Databases (2005-2009), we developed 3 methods for identifying alignments between ASCS and hospitals. The first, a geographic proximity approach, used spatial data to assign an ASC to its nearest hospital neighbor. The second, a predominant affiliation approach, assigned an ASC to the hospital with which it shared a plurality of surgeons. The third, a network community approach, linked an ASC with a larger group of hospitals held together by naturally occurring physician networks. We compared each method in terms of its ability to capture meaningful and stable affiliations and its administrative simplicity. Results: Although the proximity approach was simplest to implement and produced the most durable alignments, ASC surgeon's loyalty to the assigned hospital was low with this method. The predominant affiliation and network community approaches performed better and nearly equivalently on these metrics, capturing more meaningful affiliations between ASCs and hospitals. However, the latter's alignments were least durable, and it was complex to administer. Conclusions: We describe 3 methods for identifying natural alignments between ASCs and hospitals, each with strengths and weaknesses. These methods will help health system managers identify ASCs with which to partner. Moreover, health services researchers and policy analysts can use them to study broader communities of surgical care. Copyright (C) 2016 Wolters Kluwer Health, Inc. All rights reserved.