Nowadays, theresearchersintoControlTheoryanditsApplicationsaswellas Matrix Analysisare well awareand recognizethe importanceof PositiveS- tems. This volume contains the proceedings of the Third Multidisciplinary Symposium onPositiveSystems: Theory andApplications (POSTA09) held in Valencia, Spain, September 2 4, 2009. At present, this is the only world congress whose main topic is focused on this ?eld. After this third event, we thinkthatwehaveestablishedthebasisofaregulartriennialeventsupported in the task of the previous organizing committees and hope to have met the requirements of their...