A new theoretical framework developed over the last 10 years, the Behavior Analysis in Medicine, is described to address the challenges faced by forensic and medical or psychological assessment in daily clinical practice. With an interdisciplinary and scientifically based approach that is flexible but systematic in its application, the System for Analysis of Validity in Evaluation (SAVE) is designed to incorporate a wide range of useful methods, techniques and skills drawn from different sources (forensic medicine and psychology, criminology, health law, deception detection, etc.). It is structured in three successive or non-successive phases: verisimilitude (V1), which corresponds to the analysis of the verbal content of the statement; veracity (V2), as the congruence between verbal and non-verbal, and verification (V3), the process of checking the information previously obtained. There is also an overarching phase (V0) or identity assessment, which allows the previous results to be adapted to each case. These knowledge and skills can be learned and trained, all of them have been scientifically tested and have a large number of published references to deepen each phase. In addition, this process allows for easy adaptation to each case and context, avoiding upsetting genuinely impaired examinees and going unnoticed unless the probability of fraud is high. We therefore believe that all forensic, expert witness and legal professionals, especially doctors and psychologists, whether civil servants or independent, will benefit from this way of conducting their assessments, improving the clinical relationship and the necessary management of deception when needed.