This chapter explains how the human mind perceives the environment and how that environment can mislead the observer. It discusses how military organizations and criminals use deception practices to fool analysts and investigators. The chapter provides various deception examples, as well as analytical tools and methods used to minimize their effects. Perception includes the five senses of touch, sight, hearing, smell, and taste. The object itself may attract or distract the perception of the observer. Background and surroundings may also affect perceptions. Unlike misperception, where the observer inadvertently misperceives the object due to experience or circumstance, deception is a deliberate effort to alter the perception of the observer. Deceptions can be elaborate or quite simple and obvious, therefore, one might think they should be relatively easy to detect. Personal, or self, deception is a process of denying or rationalizing away the significance, factual basis, or substance of conflicting evidence or logical argument.