Key stock indices are the main barometers of the market’s health. The key stock indices are the ones that most investors pay attention to when trying to understand where the market stands. For the U.S., the key stock indices include the Dow Jones Industrial Average and the S&P 500. But the key stock indices can also include sub-groups that are crucial to a healthy market, such as the Dow Jones Transportation Index. In every country, there are key stock indices that one can follow to better understand the health of that country’s market.