High Frequency Mechanical Impact treatment (HFMI) is a post-weld treatment method that can be used to enhance the fatigue strength of welded details. The term HFMI covers several different high frequency peening techniques and equipment’s, which come in different commercial names, such as ultrasonic impact treatment (UIT), ultrasonic peening (UP), high-frequency impact treatment (HiFIT), etc. Common for all these techniques is that indenters of hardened high strength steel are used to impact and deform the steel material at the weld toe region with high frequency. This results in a considerable increase of fatigue resistance with respect to fatigue cracking from weld toe. This guideline document is the result of the research conducted on the topic at Chalmers University of Technology during the years 2015-2023. In addition to the derivation of fatigue resistance properties of HFMI-treated details, this work has resulted in a complete design methodology that can be used in the design of road and railway bridges with HFMI-treated details. The document also includes general requirements on welds before HFMI-treatment as well as recommendations for checks and quality assurance of the treatment. The bridge industry has shown a great interest in utilizing the benefits of HFMI-treatment both in the design of new bridges and for the purpose of fatigue life extension of existing ones. The aim of this document is to facilitate and support a safe application of HFMI treatment on steel and composite road and railway bridges.