Scotland came to national parks late. Many countries set theirs up around the idea of pristine wilderness, but Scotland did not have any pristine wilderness left — thousands of years of human settlement, agriculture, deforestation, sheep grazing, and 20th-century conifer plantations had reshaped every glen and hillside. As a result, even though committees in 1945 and 1990 recommended national park designation for several areas including Loch Lomond and the Trossachs, no action was taken. That changed in 1999 with the establishment of the Scottish Parliament. The National Parks (Scotland) Act 2000 was one of the new parliament's first pieces of legislation, and Loch Lomond and the Trossachs was the first park designated under it, opening on 24 July 2002. The Cairngorms followed in 2003. Two parks. For now, still only two.

Loch Lomond covers 27.5 square miles of surface area, making it the largest lake in Great Britain by that measure. It is also one of the most popular leisure destinations in Scotland — visitors come for kayaking and canoeing and paddleboarding and wake surfing, for the ferries that connect Tarbet and Inversnaid and Luss and Rowardennan, for the chance to climb Ben Lomond on the eastern shore. A 2005 poll of Radio Times readers voted Loch Lomond the sixth greatest natural wonder in Britain. The loch's beauty is famous enough to have generated a song — The Bonnie Banks o' Loch Lomond — that almost every Scottish school child can sing without thinking about it. The wallaby colony on Inchconnachan island — yes, wallabies, descendants of a 1940 introduction — is less widely known.

East of Loch Lomond lies the wooded country that gave the park half its name. The Trossachs were one of the first parts of Scotland to become a recognised tourist destination, helped enormously by Sir Walter Scott's poem The Lady of the Lake, published in 1810, which set its romantic narrative around Loch Katrine and brought Victorian tourists to the region in vast numbers. Loch Katrine still operates the historic steamship SS Sir Walter Scott, launched in 1899 and named in honour of the man who put the area on the tourist map. The wooded glens and small lochs of the area are sometimes described as a microcosm of a typical Highland landscape — the long Atlantic oakwoods and Caledonian pinewoods preserved here are some of the best surviving examples in lowland Scotland.

The park is enormous: 1,865 square kilometres, with a boundary 350 kilometres long, making it the fourth-largest national park in the British Isles. It contains 21 Munros — Scottish peaks over 3,000 feet — and 20 Corbetts (between 2,500 and 3,000 feet). Ben More near Crianlarich is the highest at 1,173.9 metres. Ben Lui above Tyndrum and Ben Lomond above Loch Lomond are both famous; the Arrochar Alps cluster in the western corner. The Breadalbane area in the north contains the villages of Crianlarich and Tyndrum and most of the highest peaks. The Cowal peninsula extends a finger of the park west of Loch Long, including Loch Goil and the Holy Loch. There are 22 large lochs, 50 rivers, and 39 miles of sea coast within the boundary.

Scotland's right of responsible access — sometimes called the right to roam — applies across all land and inland water in the country, including the national park. Walkers, campers, swimmers, and canoers have a legal right to be there. But in 2017, after years of complaints about litter and antisocial behaviour blamed on irresponsible campers, the park authority introduced byelaws restricting wild camping along much of Loch Lomond's shoreline. The byelaws were opposed by Mountaineering Scotland and Ramblers Scotland, who argued they criminalised responsible camping and that existing laws were already sufficient. The restrictions have since been extended to most of the major lochs in the park. Within those zones, campers must use designated sites and purchase permits between March and October. Outside them, the right to roam still applies. Beavers, which returned to Scotland from the existing River Tay population, were observed at Loch Achray in the Trossachs over the winter of 2017-18 — the first wild beavers in the park in centuries.