Dow Theory is a financial theory developed by Charles Dow in the late 19th century. It suggests that the stock market moves in trends, which can be identified and used to predict future market movements. The theory is based on the idea that the market reflects all available information and that price movements indicate the overall health of the economy. According to Dow Theory, there are three main trends: primary, secondary, and minor. A primary trend lasts for a long time, while secondary trends are shorter corrections within the primary trend. Understanding these trends can help investors make informed decisions about buying and selling stocks.