Migrating an existing library to Mendeley from another reference management software is generally quite straightforward. There are, however, certain things that you need to know before embarking on the migration process.
First of all, only metadata/references will be migrated, which means that any PDF files saved to your library will not be included in the migration. If you maintain a separate file on your computer for PDFs you can simply grap the file and import everything in one go. All PDFs will automatically be parsed in Mendeley and relevant metadata extracted. In the case of individual PDFs, just drag and drop the file to Mendeley Desktop.
Secondly, any folder system you may have entertained in your previous software will not be copied into Mendeley. If you wish to retain the same system, you need to export one folder at a time and place the contents in a duplicate folder in Mendeley.
For practical details on how to migrate to Mendeley from other popular reference systems, please see the relevant guide below.