This chapter provides a comparative study between recent operating systems, designed for embedded systems. Our study focuses, in particular, on systems designed for Multiprocessors implementations called MPSoC. An OS can be seen as abstract layer or an interface between the embedded application and the underlying hardware. In this chapter, we give a comparative study of main operating systems used in embedded systems. The originality of this chapter is that we specially focus on the OS ability to be optimized to support and manage a multiprocessor architecture. A multiprocessor system-on-chip is software driven and mastering the development complexity of the software part of MPSoC, is the key to reduce developing time factor. This opportunity could be reached through the use of a document giving a detailed description and analysis for criteria related to MPSoC. The wide diversity of existing operating systems, the huge complexity to develop an application specific or a general purpose, and the aggressive evolution of embedded systems makes the development of such a system a so difficult task. These considerations lead to the realization that a work that provides guidance for the MPSoC designers will be very beneficial for these communities.