From Orly Airport:
The bus services are run by Air France and RATP (Orlybus). The Air France buses takes about 30 minutes to reach Paris. They stop at Les Invalides and Montparnasse. The Orlybus leaves every 15 minutes and takes about 50 minutes to get downtown. It stops at Denfert-Rochereau (a major Metro stop).

You can also take the shuttle bus to the RER Orlyrail. Trains leave every 15 minutes and take about 45 minutes to reach Gare d'Austerlitz (train station and Metro stop). Another train, the Orlyval, links up with the Roissy Rail RER line B with trains leaving every 8 minutes.

If you do decide to take a taxi, it takes about 25-45 minutes to get downtown Paris. It will cost around $20-36 (100F to 180F).

From Charles de Gaulle Airport:
The bus services are provided by Air France and RATP. The Air France buses go to the western side of the city, and the RATP buses fo to the north and east. The Air France buses stop twice: at Porte Maillot and at Charles de Gaulle-Etoile. It takes about 40 minutes to make the trip and each stop has access to the Metro. There is also a bus service to Montparnasse TGV train station leaving every hour.

The RATP bus No. 350 goes to the Gare du Nord and Gare du l'Est. No. 350 goes to the Place de la Nation. Each takes about 60 minutes and leaves every 15-20 minutes.
The Roissy Rail RER train is linked by a shuttle bus from the terminals. Trains leave every 15 minutes and take about 35 minutes to get to Paris at the Gare de Nord.

Taxi lines are often long and will cost anywhere from $30-50 (150F-250F).