By now you have likely seen lots of pictures of the aftermath of the Westminster attacks.
They show members of the emergency services, off-duty doctors, MPs and members of the public rushing to help.
The image above shows a group treating Khalid Masood, the man who caused the carnage.
We've taken a closer look at who is here, what they are doing and how they worked together.
Dr Jeeves Wijesuriya is a junior doctor who was off duty at the time of the attack. When he heard screams, he ran to help the victims.
He was the first doctor at the scene and performed emergency treatment on two injured people. He also tried to save the attacker.
Dressed in blue and seen treating the suspect, these officers work in high-risk policing environments. They are police officers first and medics second.
They are trained in advanced first aid and and try to keep patients alive until other medical staff arrive.
These workers cover situations where it wouldn't be safe or practical for a normal ambulance crew to arrive.
That includes scenarios where guns may be involved, such as police raids.
These are the two people dressed in green. They've been praised for "selflessly running towards danger".
Medics have ethical rules they must follow and that means treating all patients fairly and with respect, whatever their actions.
Ethics aside, a living suspect is also more useful to police because officers have someone to question - and potentially take through the justice system.
Parliament is staffed by officers from the Parliamentary and Diplomatic Protection command.
The unit also provides protection for places like embassies and official residencies.
Most of these officers are armed. It was an officer from this unit who shot dead Khalid Masood.
PC Keith Palmer was not - and nor are some of the other officers pictured.
They are the eyes and ears on the ground and provide a face of policing and security to the public.
Former Staff Sgt Tony Davis (left) and Captain Mike Crofts (right) were among the first to rush to help PC Keith Palmer.
They were leaving a parliamentary meeting on boxing when they jumped a fence to give the policeman first aid.
Tony Davis is now a Team GB boxing coach. He said he could tell the officer was in "a great deal of difficulty" and that it was his "instinct" to help him.
Foreign Office Minister Tobias Ellwood has been called a "hero" after he was seen giving first aid to PC Palmer.
He's a Conservative MP and former military officer. He performed mouth-to-mouth resuscitation.