This practice note explains the due diligence process for an unregistered offering of debt securities that relies on Rule 144A and/or Regulation S under the Securities Act of 1933. Due diligence is a critical component of the offering, as it helps the initial purchasers and the issuer to assess the legal, business, and reputational risks, ensure accurate disclosure, and minimize the liability for material misstatements or omissions. The practice note discusses the factors that affect the extent of due diligence required, such as the nature of the issuer, the type of securities, and the ratings of the securities. The practice note also provides guidance on how to organize and conduct the due diligence review, including the roles and responsibilities of the parties involved, the use of data rooms, the preparation and response to the due diligence request list, the review of the documents and information provided by the issuer, the due diligence calls and bring-down calls, and the delivery of comfort letters, legal opinions, and officer certificates. The practice note emphasizes the need to customize the due diligence process to the specific situation of the issuer, the offering, and the relevant risk factors.