Impurities in drug substances and drug products have been important regulatory issues in the Office of Generic Drugs by having significant impact on the approvability of Abbreviated New Drug Application (ANDAs). This review begins with a discussion of ANDAs and its similarity/differences with NDAs, highlighting the importance of control of pharmaceutical impurities in generic drug product development and regulatory assessment. An overview of the FDA draft guidance documents "ANDAs: Impurities in Drug Substances" and "ANDAs: Impurities in Drug Products" are provided. This introduces the identification and qualification procedures for ANDAs and approaches to the establishment of acceptance criteria for both drug substance and drug product. Case studies included in this review illustrate the proposed pathway for determination of impurities and their acceptance criteria, based upon the general principles of these guidances.