The Lewis and Clark Centennial Exposition, commonly also known as the Lewis and Clark Exposition, and officially known as the Lewis and Clark Centennial and American Pacific Exposition and Oriental Fair, was a worldwide exposition held in Portland, Oregon, United States in 1905 to celebrate the centennial of the Lewis and Clark Expedition. While not officially considered a World's Fair by the Bureau of International Expositions, it is often informally described as such; the exposition attracted both exhibits and visitors from around the world. During the exposition's four-month run, it attracted over 1.6 million visitors, and featured exhibits from 21 countries. Portland grew from 161,000 to 270,000 residents between 1905 and 1910, a spurt that has been attributed to the exposition.^[who?]

Since its founding in 1845, Portland had evolved into a major economic center, largely fueled by the arrival of the railroads. Three transcontinental railroads used Portland as their Pacific coast terminus – the Northern, Southern, and Union Pacific Railroads. Meanwhile, Portland's wheat and flour industries were growing at an amazing rate, and Portland held "the largest flour mill on the Pacific coast." The unparalleled timber industry continued to grow, as "Oregon is second [in wooded area], with 54,300 square miles" and "in quantity of standing lumber, Oregon leads the Union, with 300 billion feet ..." Oregon's shipping was growing, too, fueled by a $1.5 million project to dike and dredge the Columbia River. During this time, Oregon's population grew from 13,294 in 1850 to 413,536 in 1900, a 3,000-percent growth, compared to the 1000-percent growth of the nation as a whole.

Despite all these positive factors, though, Oregon was not unaffected by the nationwide Long Depression, which had particular effect in 1893. Jobs were lost across the country as railroads grew too fast on a weak banking system and agricultural values fell. The state's elite business leaders all attempted to devise plans to boost the economy. Dan McAllen, a dry goods merchant, suggested in 1895 "that Portland mark the new century and pull itself out of its economic slump by holding some sort of international fair." Since the area's focus was on other issues, his proposal went unnoticed for a few years. The idea of a fair came up again occasionally, but no concentrated effort was made for various reasons. It was not until mid-1900 that this sort of action began, when "J.M. Long of the Portland Board of Trade put together a provisional committee" to begin planning some sort of fair.

Soon, a permanent board was conceived, and the head of the Portland General Electric Company, Henry W. Goode, became the president of the Board of Directors. Others included I. N. Fleischner, First Vice-President; Oskar Huber, Director of Works; Colonel Henry E. Dosch, Director of Exhibits; Henry E. Reed, Secretary; and J. A. Wakefield, Director of Concessions and Admissions. These were some of Portland's most wealthy and powerful men, working together to create an event of unmatched grandeur and power.

Although the true motivation for the fair came from an economic and business standpoint, it was still crucial to have a theme for publicity and décor. The theme for the Portland fair came from the advice of the Oregon Historical Society. They suggested that the centennial anniversary of the Lewis and Clark Expedition's stay in Oregon would be a perfect event to commemorate. As the directors wanted to include their dreams of economic growth as well, they combined the two ideas into a title that "summed up the dual goals of historic commemoration and regional boosterism: 'The Lewis and Clark Centennial and American Pacific Exposition and Oriental Fair.'" In addition, a motto was decided on, to focus the festivities and bolster publicity: "Westward The Course of Empire Takes Its Way."

Once a theme was set, the men began securing exhibits and getting support for their investments. Getting government backing was crucial due to the personal investments already made: "The Ladd and Tilton Bank [invested] $20,000, the Northern Pacific Railroad another $20,000, and brewer Henry Weinhard $10,000." Many of the substantial investments were from hotels (the Imperial Hotel Company purchased 50 shares worth $5,000), restaurants, streetcar companies, and retailers – all groups with much to gain from the success of the fair and the economic prosperity it could provide. In addition, approximately 3,000 average citizens purchased stock certificates both as investment opportunities and to support what they viewed as a worthwhile venture. These individual investments eventually paid off greatly.

Seeing the potential benefits of the fair's success, the state legislature began planning appropriations for the fair. Although they "had [little] interest in the historical heroes and their 2,000-mile (3,200 km) trek ... they [shared] the vision of Pacific trade that had motivated the exploration and settlement of the Oregon Country." Thus, the Legislature passed "An Act Celebrating the One Hundredth Anniversary of the Exploration of the Oregon Country," which appropriated a sum of approximately $500,000 for the fair. After the Fair's completion, it was reported that

The money expended by the two departments amounted to about four hundred thousand dollars each, the State appropriating that expended by the Commission, while the stockholders of the Corporation subscribed about an equal amount. The proceeds from the Exposition were expended entirely under the direction of the Corporation. The government of the United States appropriated $475,000 and about an equal value in exhibits, the exact amounts of which I am unable to give.