Motivational Interviewing (MI) is a client-centered, directive approach for behavior change. Self-Determination Theory (SDT) is a theory of human motivation and self-determined growth. Both of these social-cognitive theories aim to explain motivation and health behavior change and have generated a lot of research on how to increase human motivation. However, MI lacks a definite theory on mechanisms of behavior change and SDT offers a theory of motivation based on extensive research on human interactions. This paper aims to critically look at facilitators of client motivation for behavior change in-session from a social-cognitive perspective on one hand, and neural correlates related to client behavior change on the other hand. MI and SDT somewhat similarly explain what most importantly determines client motivation for change in-session. However, SDT mainly focus on psychological needs such as fostering client autonomy, and MI focus on the therapeutic alliance and on generating client change talk in-session. Efforts to bridge the two methods aim at generating a clearer definition of motivation in MI, and a better framework of practice in SDT. Studies on neural correlates of behavior change support and challenge elements of both approaches, indicating the importance of autonomy and relatedness for motivating positive behavior change.