According to Wikipedia, Managed Pressure Drilling (MPD) is defined by the International Association of Drilling Contractors (IADC) as "an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore." The objectives of MPD are "to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly."
For offshore drilling, there are two main technologies on the market to achieve MPD:
Controller Mud Level (CML)
Gradient Drilling Solutions provides CML service.
Surface Back Pressure (SBP)
Almost all major oil service companies provide SBP service, for both onshore and offshore drilling