This Practice Note explains 10b-5 letters delivered to underwriters by issuers’ and underwriters' counsel in connection with offerings of securities in the United States and covers: (1) US federal securities law liability and the due diligence defence, (2) contents of a 10b-5 letter (definition of scope of offering documentation, description of scope of activities undertaken, disclaimer of independent verification, negative assurance and exclusions),and (3) impact of 10b-5 letters on non-SEC-registered securities offerings.