The big data era, has made it more diffi cult than before, for us to construct hypotheses. However, the role of hypotheses is increasingly more important. In this chapter, the essence of hypotheses in the big data age will be explained fi rst. Then classical reasoning forms, such as induction, deduction, and analogy, will be discussed as fundamental techniques for constructing hypotheses. After that, abduction as plausible reasoning and causalities and correlations as basic concepts related to reasoning will be summarized.