Quarterly panel data covering the largest 10,000 city-pairs in the domestic US from 1998 are used to investigate airlines market entry and exit decisions. Several models are estimated looking at changes in the number of carriers serving in a market. The influence of a number of markets characteristics is examined, including number of passengers, average fare, average yield, service concentration, great circle distance, and seasonality. The results suggest that airlines are more likely to enter a market when market concentration is high and there are high average fares. Also airlines tend to enter new markets in the second quarter, then in the fourth quarter, and then in the third quarter of the year.