If you have data onyour contacts in another system than DI, you can generally export these data in Excel or .csv format. You can then upload the file to DI to import your contact data.
Prerequisites
If you are using a new project and it is your first import, make sure to set your project field structure before.
Note: Importing contacts does not generate automatically a contact group for targeting. However, you can use an import as a target criterion to create a Group.
Step 1: Prepare the Import File
The file to import must be Excel (.xls or .xlsx) or .csv format (comma separated values). The size of the file must not exceed 50 Mb. This file can be compressed to the ZIP format if needed (not locked with a password and containing only one file).
To learn more, read the guide on accepted formats.
Clarification about consents
If you have implemented a consent center and wish to keep a record of consent dates to your communications, it is important that the contact file you want to import includes columns to hold the subscription fields and dates for each of your communications. If you do not add those columns, your contacts will not be shown as subscribers and therefore, messages won't be entitled to be sent to them.
As an example, if you have a newsletter, you should have at least the following:
- 1 column name Newsletter, in which you will enter either 1 for Subscribed, 0 for Unsubscribed or leave the value empy if you do not know.
- 1 column Newsletter_Optin_Date, to hold the date of the opt-in/out, in the proper format.
Step 2: Upload the Import File
Go to the Contacts section and click on Import:
Select Import from file:
Then, select Basic information about my contacts:
For the other data import options, you need relational tables or an e-commerce integration. See the article on data import.
Then, you have to choose which mode you would like to use. We strongly recommend the Simplified mode since it offers more feedback and guidance (the rest of this article explains this mode). 
Upload your file:
Make a choice among the import actions:
Note on the primary key: When importing contacts, the system determines whether it should update or add a contact based on the information contained in the field set as the primary key (often the email address). For example, if you choose Add and update contacts, and your primary key is the email, the system will add all contacts from the file that do not have their email address already in the project and will update all the contacts that have a matching email address in both the source file and the current project.
Step 3: Map the Fields
You must now map the destination of your Excel or .csv file data. In the center column, select the fields you want to use to receive the data in your project:
If there is a column you do not want to include in the import, leave the default option (Not imported). If a field is defined as required in your project, it needs to be mapped every time you make an import.
To reuse the same template for future imports, you can check the following option:
Next time you make an import, you won't have to configure the mapping again if you select the template displayed under the option Apply a mapping template:
About consents
If you have a consent center in your project, it is important that you include in your file fields for opt-ins and dates of opt-ins so you can map them to the corresponding fields in your project. Otherwise, your contacts won't be subscribed.
Step 4: Validate the Mapping
Now that the system processed your mapping, you need to check if everything is in order. If there are errors, consult the details to learn how to correct them: 
You can either reimport the file with the corrections or ignore the errors. If you decide to ignore errors, the data in error won't be imported.
If you don't have errors, proceed with the import.
Results
You can see the recent imports in the history:
The server regularly performs a data cleaning. Thus, you may not see older imports in the list.
You now have a list of contacts in your project that you can use to create Groups.