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.
Step 1: Prepare the Import File
The file to import must be Excel (.xls or .xlsx) or .csv format (comma-separated values). 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).
Note: Most date formats are compatible (see step 3).
Clarification about consents
To import consents, your import file must include a column for each opt-in type. For example, if you have a newsletter, you could include in your file a column, in which you will enter either 1 for Subscribed, 0 for Unsubscribed or leave the value empty if you do not know:
It is not required to include the optin date in your file (but it is recommended). If there is column for dates, you will be required, during field mapping, to select a date (the same date for every contact included in that import). For more details, see the article on importing the consents.
Step 2: Upload the Import File
Go to Audience → Contacts and click 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.
| Date fields Most date format are supported. You must indicate the format used among the options in the list (at the bottom of the page):  If the format in your file is not in the options, you can select Other and indicate manually a standard format |
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 your file includes consents, you must map the optin field and the opt-in field created in your project:
Note: the system generates automatically opt-in date fields when you create an opt-in field.
If your file does not have opt-in dates, you will need to indicate (at the bottom of the page) a date, which will be the same for all contacts included in this import:
See the article on importing consents.
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 segment with groups. Each import generates a group with the contacts imported.