Montana has not had a single reliable economic industry throughout the 19th and 20th centuries. From the earliest trappers and miners to today’s tourists and oil workers there have been a series of boom and busts. The primary sector jobs in extractive industries – mining, oil, agriculture, and timber – have built the economy to record highs with corresponding downturns. Agriculture remains a constant although federal policies and the rise of corporate farming have changed the industry considerably. Tourism had its start after World War II with the advent of the interstate highway system. Timber has faced challenges from environmental groups.