A surprising number of students in psychology, behavioral science, and related social science classes fail to appreciate the importance of biological mechanisms to understanding behavior. To help teachers promote this understanding, this paper outlines six sources of evidence. These are (a) phylogenetic, (b) genetic/developmental, (c) clinical, (d) experimental, (e) brain recordings, and (f) experiential. The evidence can be easily understood from common knowledge or personal experience and can be incorporated into an introductory level lecture. It is concluded that the successful study of human behavior requires the student to appreciate the biological basis of the discipline.