This chapter will describe scientific principles and best practices for the assessment of drug product leachables, and will cov- er various important concepts, including: 1) the requirement for leachables studies; 2) fundamental concepts for leachables studies; 3) the basis of thresholds for leachables and general guidance about application of these thresholds; 4) design and implementation of leachables studies; 5) leachables method development and validation; 6) correlation of results from extract- ables assessments and routine extractables testing with leachables studies; and 7) establishment of leachables specifications including acceptance criteria.