AD Connect (also known as AD Sync or Directory Sync)

Though this technology has been rebranded many times, the concept is essentially the same as it was several years ago. You install a lightweight version of a Forefront Identity Manager (FIM) server on one of your domain computers. This service runs every few hours, reading both Windows AD and Azure AD to synchronize changes between the two.

Due to its complexity, and its history of needing significant maintenance, we do not generally recommend AD Connect to organizations of less than 50 people. However, it is important to point out that AD Connect has significantly improved since its previous versions, AD Sync and DirSync. Ultimately, you will need to decide based on how you want to perform user management tasks, your security needs, and other considerations.