The impact of the walkout will vary by which part of the country you live in.
The first thing to bear in mind is that under trade union laws striking staff have to ensure life-preserving care is provided.
While that is not strictly defined, the highest category calls - the immediately life-threatening ones where patients are in cardiac arrest or not breathing - should be covered.
Category two calls, which include things such as heart attacks and strokes, may only be partially covered. It could mean patients who have had falls not getting an ambulance response.
To ensure life-preserving care is provided union members agree to cross the picket line and work under a system known as derogation, while remaining supportive of strike action.
The second issue is that the two unions do not have a strike mandate for every service in England and the Welsh Ambulance Service.
In Wales only GMB members will walkout. In England Unison has a mandate in half of the 10 services and GMB in eight. Only the East of England will remain completely unaffected.
Between them the two unions represent around two-thirds of ambulance staff.
Ambulance staff in Scotland and Northern Ireland are not on strike.
Ambulance staff on strike in large parts of England and all of Wales
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Welcome to our rolling coverage as ambulance staff go on strike in some areas of England and across the Welsh Ambulance Service.
Crews last went on strike in December but this time more staff are joining the action - not just 999 response crews.Services in many parts of England and all of Wales will take industrial action over pay and staffing.
Leaders from the Unison union say the strike is a direct result of the government's refusal to negotiate over pay.
Life-threatening calls to 999, as well as the most serious emergencies, will still be responded to, they say.
Live Reporting
Edited by Heather Sharp
All times stated are UK
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What is the impact likely to be?
Nick Triggle
Health Correspondent
The impact of the walkout will vary by which part of the country you live in.
The first thing to bear in mind is that under trade union laws striking staff have to ensure life-preserving care is provided.
While that is not strictly defined, the highest category calls - the immediately life-threatening ones where patients are in cardiac arrest or not breathing - should be covered.
Category two calls, which include things such as heart attacks and strokes, may only be partially covered. It could mean patients who have had falls not getting an ambulance response.
To ensure life-preserving care is provided union members agree to cross the picket line and work under a system known as derogation, while remaining supportive of strike action.
The second issue is that the two unions do not have a strike mandate for every service in England and the Welsh Ambulance Service.
In Wales only GMB members will walkout. In England Unison has a mandate in half of the 10 services and GMB in eight. Only the East of England will remain completely unaffected.
Between them the two unions represent around two-thirds of ambulance staff.
Ambulance staff in Scotland and Northern Ireland are not on strike.
Ambulance staff on strike in large parts of England and all of Wales
Welcome to our rolling coverage as ambulance staff go on strike in some areas of England and across the Welsh Ambulance Service.
Crews last went on strike in December but this time more staff are joining the action - not just 999 response crews.Services in many parts of England and all of Wales will take industrial action over pay and staffing.
Leaders from the Unison union say the strike is a direct result of the government's refusal to negotiate over pay.
Life-threatening calls to 999, as well as the most serious emergencies, will still be responded to, they say.
Stay with us as we bring you the latest.