The S&P 500 is a stock index that tracks the performance of 500 of the leading publicly traded companies in the U.S., serving as a key benchmark for the overall health of the U.S. economy. Tech firms now make up a major share of the index and have an important impact on its movements.

Datadog shares are up 9.4% in premarket on news that the monitoring-software maker is joining the S&P 500, and will replace Juniper Networks. TripAdvisor shares are up over 6% after The Wall Street Journal reported,