Medication adherence is de ned as the extent to which medications are taken as pre- scribed, such that nonadherence is not dichotomous, but rather a spectrum of types and degrees of discordance with medication prescription (99). Estimates of signi cant nonadherence in patients with HFrEF vary from 20% to 50% (100 103), with some difference by drug (104). Such nonadherence is associated with worse outcomes in HF (105,106). In addition to nonadherence, a large proportion of patients with HFrEF do not receive target doses of medical therapies (107), even in the absence of docu- mented intolerance. Reasons for nonadherence are complex (108,109), as outlined in Table 9. Unintentional nonadherence is thought to be more common than intentional non- adherence (99,110). As Table 9 shows, the ability of pa- tients to follow treatment plans in an optimal manner is frequently compromised by more than 1 barrier (111,112). Patients with HF, especially those with HFrEF, have indications for multiple medication therapies. In addi- tion, the HF population has a rising prevalence of comorbidities that necessitate their own pharmacological therapies. As a result, patients with HF are prescribed an average of 6 different medications totaling more than 10 daily doses (113,114). Consequently, interventions that target adherence in HF must be multidisciplinary, multi- factorial, and personalized to the particular demands experienced by the patient. 5.5.2. General Approaches to Improving Adherence Regularly assessing adherence helps guide individual approaches and tailor the intensity and type of adherence interventions. Notably, however, clinicians tend to over- estimate actual adherence, and no perfect measure of adherence exists. 