[5] It is on the north bank of the Boulder River between Butte and Helena, slightly east of the Continental Divide, at the intersection of Interstate 15 and Montana Highway 69.

[6] It is part of the Helena Micropolitan Statistical Area, which includes all of Lewis and Clark and Jefferson counties; its population is 83,058 according to the 2020 Census.

In the 21st century, it is the center of government in Jefferson County, and institutions based in the town offer services for disabled adults and troubled youths.

Its library system serves about 10,000 people, and its high school district covers more than 1,000 square miles (2,600 km2).

[7] It later became a trading center for nearby agricultural areas and the Elkhorn, Comet, and Baltimore mining districts.

According to the United States Census Bureau, the city has a total area of 1.12 square miles (2.90 km2), all land.

[10] In the late Cretaceous Period (roughly 81 to 74 million years ago), molten rock (magma) rose to the Earth's surface in and near what later became Jefferson County and eventually formed an intrusive body of granitic rock up to 10 miles (16 km) thick and 100 miles (161 km) in diameter.

This body, known as the Boulder Batholith, extends from Helena to Butte, and is the host rock for the many valuable ores mined in the region.

Millions of years later, weathering allowed gold in the veins to wash down to the gravels in the Boulder River valley.

In addition, Riverside Corrections, a state-run detention center for juvenile females, employs about 30 people.

[16] Boulder and vicinity are home to commercial "health mines" for which it is claimed by testimonial that exposure to radon gas mitigates auto-immune diseases such as arthritis.

[17] Though contributing to the economy of the area by attracting tourists, the practice is controversial because of the "well-documented ill effects of high-dose radiation on the body.

In 2005, it had about 39,000 books and serial volumes in its collection, 85 subscriptions, 3,200 videos, 1,500 audio materials, and 21 Internet terminals.

[20] Boulder's Elkhorn Treatment Center, which operates under the Montana Department of Corrections, is known for its work with women battling methamphetamine addiction.

[29] A low-power radio station, KEME, 106.3 FM, owned by Jefferson County Disaster and Emergency Services, broadcasts from Boulder.