I will be showing you how to import and update a single contact from your third party system to Constant Contact using the Constant Contact API. Here is the API description for the POST (Create) call and PUT (Update) call.
Import Contacts in Bulk using Constant Contact Email Marketing API in ASP.NET (C# and VB.NET)
In this post I will show you how to use the Bulk Activities from the Constant Contact API to import contacts from your custom application to your Constant Contact Account.