This document provides guidance on best practice for in-situ measurement of underwater sound, for processing the data, and for reporting the measurements using appropriate metrics. Measured noise levels are sometimes difficult to compare because different measurement methodologies or acoustic metrics are used, and results can take on different meanings for each different application, leading to a risk of misunderstandings between scientists from different disciplines. Acoustic measurements are required for applications as diverse as acoustical oceanography, sonar, geophysical exploration, underwater communications, and offshore engineering. More recently, there has been an increased need to make in-situ measurements of underwater noise for the assessment of risk to marine life. Although not intended as a standard, these guidelines address the need for a common approach, and the desire to promote best practice.