The main objective of this study is to identify the eligibility criteria for treatment with lithium in BDI in terms of response, safety and tolerability. R-LiNK proposes a pragmatic study of individuals with BDI when lithium treatment is initiated, to identify early biomarkers to stratify individuals with BDI according to their response to lithium assessed prospectively (two years). The detailed baseline assessment before and after lithium initiation includes extensive clinical evaluation with measures of blood “omics”, anatomic/structural Magnetic Resonance Imaging (MRI) and 7Li-MRI derived-markers. These markers will be tested as predictors of the response status at the endpoint (2 years). Translation will be assessed in terms of positive and negative predictive values of markers, the utility of markers when employed alone or in combination (i.e. additivity or redundancy), patient acceptability and cost-effectiveness. Because, monitoring the adherence to consistently taking the medication is critical, we will introduce an interactive package that allows self-rating of mental state, and offer electronical reminders during the whole study period. Finally, during this project we will also develop a device that will enable self-measurement of salivary lithium levels in the person’s own home.