Humans explore their world by selecting sensor observations, fusing data, and deciding how to act. For the most part, the goal of sensory data fusion is to increase confidence in entity identification and location. Since the 4th century B.C., the fusion process has been acknowledged through sensed data association. However, in the past century, probability and statistics formalized the association problem with mathematical correlation. Together, association and correlation aid in reducing uncertainty for decision making. Fusion can have a variety of meanings, such as data fusion, sensor fusion, and information fusion. Thus, it is important to explore terminology. Data fusion is the correlation of raw data whereas sensor fusion is the multimodal integration of transduced data into a common perception. Information fusion (IF) is the collection of sensory and knowledge data to create a unique understanding upon which a user performs response management. IF for decision making includes: the situational context, sensor control, and the user-machine interaction. In this chapter, we will highlight many of the key aspects of fusion research to give the reader a flavor of what IF can do and cannot do to aid a user. This chapter will discuss the fundamentals of information fusion: (1) who: decision makers and machines; (2) what: data, sensor, and information; (3) where: applications; (4) when: appropriate situations; (5) why: to reduce uncertainty and increase confidence; and (6) how: techniques. What you gain from reading this chapter is a perspective of how to integrate the fusion-machine and the cognitive-user, a taxonomy of information fusion algorithms, and an appreciation of the benefits and limitations of fusion designs. Sensor and data fusion books are useful for the reader who is unfamiliar with basic concepts. This chapter is not intended to repeat the explorations of these texts, but to summarize important aspects of IF relative to “cognitive fusion” which integrates the user with the fusion machine for decision making.