Police have released the CCTV image of four men they want to speak to after a pair were attacked in Watford town centre.
Officers said a man in his 20s and a friend were walking along Beechen Grove, near to the junction in Loates Lane, at around 3am on Saturday when they were assaulted.
One of the men was hit in the back of the head causing him to fall.
The attackers then made off towards the town centre.
PC Lauren Shipman said: "I am appealing to anyone who may recognise the people pictured to contact police with information. I am also appealing to two men who stopped and spoke to the victim and his friend.
"Both men are described as white, in their early 20s. One of the men was walking a dog. I am appealing to these men to get in contact with us, as they can help with our investigation."
Anyone with information is asked contact PC Lauren Shipman on 01923 472426 or via the Herts Police non-emergency number 101.
Alternatively Crimestoppers can be called anonymously on 0800 555 111.