Scientific investigation from the last two decades of physical, social, emotional and cognitive benefits of human-animal interactions has led to the emergence of a new domain of inquiry and intervention: Animal Assisted Therapy (AAT). AAT is nowadays a common technique in health-improving programs in humans. Everywhere in the world, there are high costs associated with the treatment of mental disorders. As a consequence, there is a great interest in less expansive therapy programs such as AAT in general and more specific in dog-assisted therapy. However, until now there is no convergent arguments regarding psychological and behavioral variables which should be carefully considered when AAT is incorporated in psychotherapy protocols (in terms of achieving a maximum efficacy). Our study aims to identify these variables by reviewing recent studies with controlled clinical trials involving AAT. Studies were selected from two international databases (PsychInfo and ISI Web of Science, 1990 - 2008) using Keywords and combinations of them such as animal assisted therapy and pet therapy. Our study shows that when using AAT in psychotherapy it is important to take into account the following variables: self-esteem, self-control, prosocial behavior, orientation toward reality and the ability to express emotions in AAT context.