Paris is served by two main airports, Roissy-Charles-de-Gaulle to the northeast and Orly to the south, both roughly 30 kilometers from the city center. The trip is easy by public transit, but the best option really depends on your arrival terminal, your budget and how much luggage you're carrying.
From Roissy-Charles-de-Gaulle (CDG)
The RER B train connects all three terminals directly to central Paris (Châtelet-Les Halles, Saint-Michel, Gare du Nord) in 35 to 50 minutes depending on the time of day. It's the most reliable option since it doesn't depend on road traffic. The Roissybus and shuttle buses also serve the Opéra area, but allow extra time during rush hour.
From Orly
The airport connects to the metro (line 14) via the automated Orlyval shuttle, with a possible transfer to the RER B at Antony station. Expect around 30 to 40 minutes total to reach the center depending on your final destination.
The RER B, the backbone of the trip
This line connects both airports to the heart of Paris and runs from early morning until well past midnight. Buy your ticket before boarding (machines are available at every station) and keep it until you exit, checks are frequent.
Tips
A taxi is still the simplest option with heavy luggage or late at night: flat fares exist between each airport and either side of the Seine, just confirm the price before getting in. During transit strikes, always allow at least an extra hour.
