Increasing the level of autonomy allows reducing reliance on the human supervisor. Addressing autonomy in the real world with unknown events, it is strongly critical to instantaneous adaptation to the continuously changing situations. Autonomous adaptation relies on the understanding of the surrounding environment. Complicated missions that cannot be accurately defined in advance will need to be resolved through intermittent communication with a human supervisor. This will restrict the applicability and accuracy of such vehicles. A fully autonomous vehicle should have capability to consider its own position as well as, its environment, to properly react to unexpected or dynamic circumstances. This chapter aims to provide a general background of some Unmanned Vehicles (UVs) and existing difficulties on the way of having a true autonomy in their applications. Assessing the levels of autonomy and its related properties in internal and external situation awareness toward robust mission planning are also discussed in this chapter.