The purpose of this research study is to characterize the mechanisms contributing to cognitive impairment and accelerated cognitive decline in Late Life Depression (LLD). This is a non-randomized, observational, non-treatment study that originally launched in 2015, enrolling 133 participants. From the originally enrolled participants, the continuation of the ADNI-D study will enroll 120 participants which will include following participants from the original (parent) protocol and enrollment of new participants for a period of 30 months. Data from an additional 300 non-depressed subjects will be used from ADNI studies for comparison. Depression history, symptom severity and health information will be collected at the initial visit to determine eligibility. An magnetic resonance imaging (MRI) scan, as well as amyloid (florbetapir) and tau (flortaucipr) positron emission tomography (PET) imaging will be conducted at San Francisco VA. Collection of plasma and serum for biomarkers, clinical assessments and cognitive assessments will be conducted at two time points. Blood samples will also be collected for genetic analysis.