This document is an update of the document titled 'Guidance on Evaluation, Resolution, and Documentation of Analytical Problems Associated with Compliance Monitoring', which was published in 1993. The 1993 document has been referred to as the Pumpkin Book because of its pumpkin-colored cover. The material and technical advice in the Pumpkin Book, and this document are based on questions and situations directed to EPA's Clean Water Act chemists by the EPA regions, state agencies and other users of our methods. The questions and situations discussed in this document concern analytical challenges encountered in the conduct of compliance monitoring under the Clean Water Act (CWA) for chemical pollutants. The purpose of this document is to recommend ways to document the existence of a matrix or analytical problem with a CWA sample analysis, and mitigate these problems. This document is organized as follows: Chapter 1 Introduction; Chapter 2 Sampling Requirements; Chapter 3 Flexibility to Modify an Analytical Method; Chapter 4 Data Required to Document Matrix Interference; Chapter 5 Case Histories of Reports of Matrix Interferences; Chapter 6 Solutions to Matrix Interference Problems; Chapter 7 Review of Data from Analysis of Samples; Chapter 8 When a Matrix Interference Is Demonstrated; Chapter 9 Sources of Additional Help.