Depression in older adults has been detected, diagnosed and treated more frequently in recent years. However, substantial gaps in effective treatment remain. Adherence to depression treatment can be viewed as the 'next frontier' in the treatment of late-life depression. Using the Theory of Reasoned Action, a model of health behaviours, this paper conceptualizes and reviews the current evidence for key patient-level factors associated with depression treatment adherence among older adults. We categorize these factors according to how their impact on adherence might be affected by specialized treatment approaches or interventions as: (i) modifiable; (ii) potentially modifiable; and (iii) non-modifiable. Based on current evidence, modifiable factors associated with depression treatment adherence include patient attitudes, beliefs and social norms. Patient attitudes include perceptions of the effectiveness of depression treatment, preferences for the type of depression treatment and concepts regarding the aetiology of depression (e.g. resistance to viewing depression as a medical illness). There is also evidence from the literature that spiritual and religious beliefs may be important determinants of adherence to depression care. Social norms such as the impact of caregiver agreement with treatment recommendations and stigma may also affect adherence to depression treatment. Other factors may be less modifiable per se, but they may have an impact on adherence that is potentially modifiable by specialized interventions. Based upon a review of the current literature, potentially modifiable factors associated with adherence to depression treatment include co-morbid anxiety, substance use, cognitive status, polypharmacy and medical co-morbidity, social support and the cost of treatment. Finally, non-modifiable factors include patient gender and race. Importantly, non-modifiable factors may interact with modifiable factors to affect health behavioural intent (e.g. race and spiritual beliefs). Thus, adherence to depression treatment in older adults is associated with multiple factors. Strategies to improve patient adherence need to be multidimensional, including consideration of age-related cognitive and co-morbidity factors, environmental and social factors, functional status and belief systems. Evidence-based interventions involving greater patient, caregiver, provider and public health education should be developed to decrease stigma, negative attitudes and other modifiable barriers to detection, diagnosis, treatment and adherence to depression treatment. These interventions should also be tailored to the individual as well as to the treatment setting. While important progress has been made in increasing detection of depression in older adults, greater focus now needs to be placed on treatment engagement and continuation of improvements in quality of life, reducing suffering and achieving better outcomes.