I thought I would update this with some more information since I get this question a lot:
http://developers.marketo.com/rest-api/
Daily Quota: Most subscriptions are allocated 10,000 API calls per day (which resets daily at 12:00AM CST). You can increase your daily quota through your account manager.
Rate Limit: API access per instance limited to 100 calls per 20 seconds.
Concurrency Limit: Maximum of 10 concurrent API calls.
For the Daily limit:
Option 1: Call your account manager. This will cost you $'s. For a client I work for we have negotiated a much higher limit.
Option 2: Store and Batch your records. For example, you can send a batch of 300 leads in a single lead insert/update call. Which means you can insert/update 3,000,000 leads per day.
For the Rate limit:
Option 1 will probably not work. Your account manager will be reluctant to change this unless you a very large company.
Option 2: You need to add some governance to your code. There are several ways to do this, including queues, timers with a counter, etc. If you make multi-threaded calls, you will need to take into account concurrency etc.
Concurrent call limit:
You have to limit your concurrent threads to 10.