The importance of the Southern Ocean to the global climate system and the uniqueness of its ecosystems are well known. The region is remote and logistically difficult to access and thus is one of the least sampled regions on the planet. Design and implementation of an observing system that encompasses physical, biogeochemical and ecological processes is therefore a formidable challenge. Building on meetings held in Hobart in 2006, Bremen in 2007 and St Petersburg in 2008 we present a draft plan for a Southern Ocean Observing System (SOOS). This document examines: (i) why sustained observations are needed in the Southern Ocean and what science/policy questions they address; (ii) what mix of observations are required to address these questions; (iii) what is presently done and possible and (iv) a vision for the future. SOOS is a key component of the SCAR Pan Antarctic Observing System (PantOS) and is co-sponsored by the Scientific Committee on Antarctic Research (SCAR), the Scientific Committee on Oceanic Research (SCOR), the Census of Antarctic Marine Life (CAML), the Partnership for Observation of the Global Oceans (POGO) and the Global Ocean Observing System (GOOS). Other organisations, in particular the National Oceanic and Atmospheric Administration (NOAA), have provided significant funding.