As organizations increase their adoption of cloud services, we see attackers following them to the cloud. Microsoft Office 365 is becoming the most common email platform in enterprises across the world and it is also becoming an increasingly interesting target for threat actors. Office 365 encompasses not only Exchange, but also Teams, SharePoint, OneDrive, and more. The sheer volume of data stored in Office 365 means that in many cases an attacker need not compromise the on-premise network to complete their mission.In this talk, we walk through a number of case studies taken from real APT intrusions that we've been a part of. We will begin with relatively unsophisticated techniques that are used by small-time actors and have been widely discussed. From there, we work our way up to the most sophisticated and stealthy techniques that we have only observed in the wild on a few occasions. These techniques utilize parts of Office 365 that are often poorly understood and not closely monitored.Along the way, we will provide insight into the various forensic artifacts available to an investigator and their many nuances. We will discuss some important gotchas that can trip up inexperienced analysts. Lastly, we will also discuss important best practices for administrators to defend their tenants against these increasingly sophisticated threats.