More than a million acres of Wales, or about a fifth of the country, has been opened up to walkers for the first time under new right to roam laws.
Walkers can now stray off public rights of way in many areas
It has increased the land open to ramblers threefold and has been hailed as a major boost for tourism.
Walkers make an estimated 33m trips to Wales each year, bringing in more than £500m.
But not all areas have been opened to walkers and landowners have urged people not to "abuse" their new rights.
The Country Landowners Association (CLA) said 36,000 landowners had not been compensated for having to allow people on their property.
The Countryside and Rights of Way Act gives walkers permission to roam over a total of 451,000 hectares, around 20% of the Welsh countryside.
This is made up of 350,000 hectares of open country, particularly high open moorland, as well as common land and public forestry.
Much of the land is accessed walking away from the 25,000 miles of existing public rights of way.
The extra areas are clearly marked on new Ordnance Survey maps.
Walkers must control their dogs during the lambing season
Environment Minister Carwyn Jones said: "This is a momentous day for Wales. It stands to reason that local businesses, communities and farmers all stand to benefit from increased recreational use of the countryside."
CLA Wales director Julian Salmon said: "We are concerned that visitors to the countryside find out where they can and can't go before setting off, and what they can and can't do once they get there."
He said the new rights did not constitute a general right to roam anywhere.
"These areas of land still belong to the farmers and landowners." said Mr Salmon.
"They still have to pay for its upkeep, maintenance and conservation, as well as try to make a living from farming these areas."