Integrity diagnostic has become one of the key prerequisite criteria when dealing with uncertainties in the well completion of mature assets. This is particularly true in Malaysia, where most of the tubing and casing in production wells has aged more than 30 years, and yet some of these wells are still producing beyond their original design life expectancy. Locating the source of integrity issues relies on being able to identify leaks and/or flow behind casing. Typically, conventional technologies have been used for anomaly detection, including spinners and temperature logs, downhole cameras, calipers and noise logs. In general these lack accuracy, with smaller leaks being particularly difficult to identify. Development studies pioneered in the well logging industry have led to a new diagnostic approach utilising high definition ultrasound technology. This has created a fast, accurate and reliable technique which can be used to identify leaks as small as 0.02 litres per minute inside the completion tubing. Field D and Field BN fields are two of PETRONAS Carigali Sdn. Bhd. (PCSB) mature fields where ultrasound technology has been implemented aggressively in order to mitigate complex completion problems involving tubing and annulus leaks. PCSB has successfully implemented this technology since 2008; to date the company has resolved more than 100 wells uncertainties using the method to determine multiple completion leaks and flow channelling behind casing. The ultrasound-based leak detection case histories demonstrate the practicality of the technique in many cased-hole applications. The related ultrasound-based flow-behind-pipe survey detects liquid or gas flow behind casing with a measurement from within the tubing. This unwanted flow can have a variety of negative outcomes, including poor well performance, ground water or reservoir contamination and/or more catastrophic uncontrolled fluid escape at surface. The diagnostic is sensitive to the ultrasound energy created by turbulent liquid or gas flow through small restrictions in annuli. From within the tubing, it can detect and locate undesirable flow behind pipe with great accuracy - even through multiple casing strings. This paper shares some experiences and best practices developed while implementing the technology in Field D and Field BN fields.