Background: Performance-measurement systems may work best when they account for the reasons why physicians do not provide guideline-recommended interventions. Objective: This article develops a conceptual framework for understanding the proximate, patient-centered reasons why physicians do not prescribe angiotensin-converting enzyme (ACE) inhibitors or β-blockers to patients with heart failure and reduced systolic function. Methods: This was a focus group study using a 2-stage design. Academically affiliated clinicians of different specialties and levels of training were recruited by e-mailed invitations sent to clinicians within each target group. To be included, candidates needed to be currently practicing in an ambulatory care setting in which they encountered patients with heart failure. In the first part of each group, participants were asked to describe reasons for not prescribing ACE inhibitors or -blockers for patients with heart failure. Next, participants were asked to develop concept maps that organized these reasons into categories and described the relationships between these categories. The concept maps from each group were synthesized to develop a consensus scheme for categorizing reasons for nonprescribing. Results: There were 31 participants in 7 focus groups; median age was 31 years and 55% (17/31) were women. Two broad themes emerged. First, clinicians hinted at their own attitude-related barriers to prescribing. However, they framed their comments largely in terms of patient-centered reasons for nonprescribing that arose in individual patient encounters. Second, decision making about heart failure drug therapy often involved a complex and overlapping series of considerations. Five categories of reasons for not prescribing ACE inhibitors or -blockers emerged: (1) adverse effects of drug therapy; (2) nonadherence to therapeutic and monitoring plan; (3) patients' preferences and beliefs; (4) comanagement and transitions of care; and (5) prioritization and patient benefit. Conclusions: Physicians' reasons for not prescribing guideline-recommended drugs for heart failure are complex but can be organized into a useful taxonomy. This taxonomy may be helpful for performance-measurement and quality-improvement programs that seek to understand reasons for physicians' nonadherence to guidelines. © 2010 Elsevier HS Journals, Inc. All rights reserved.