Abstract : The current acceleration of technological and economic evolutions brings about more and more frequent changes which cause instability. Companies have to change accordingly, so as to remain competitive. This instability affects work contracts and modifies the balance of the employer /employee relationship. Hence, employees must also be able to adapt the resources which they provide to employers in order to preserve their usefulness or, more largely, their employability on internal or external labour markets. It is the condition to secure perpetuation of an existing work contract or additional contracts. Among various elements structuring this employability, knowledge and the dynamic which determines its progression in synchrony with the needs of the market, play a major role. After theoretically defining the characteristics of the work expected and provided and empirically analysing the decisional factors upon which managers make decisions in ten industrial companies surveyed on their recruitment, evolution, evaluation, training, reclassification, redeployment, or even employee dismissal procedures, we are lead to suggest a new structuring of major choice criterions and to advise employers to use the mental employability concept as decision criterion for employee assignment, i.e. all decisions establishing a link between a job and a person. To promote this process, we propose a decision-making tool based on mental employability evaluation. This aims to characterize the optimal ratio between employer requirements - specified with regard to the targeted post - and resources provided by the employee - knowledge, abilities as learning one and personality - if working conditions proposal suits him. The basis of this approach is the employers' points of view and it unfolds in six stages: The first five stages are based on knowledge requirements and personality, they measure objective employability, compared to the targeted post, in order to identify nature and importance of possible negative or positive variations, so as to, make the best choice or plan on relevant corrective actions according to the type of decisions to be taken. The sixth stage determines subjective employability, assessed according to both sides' demands in order to check possible circumstances incompatible with the signature of the proposed work contract. Four examples are used in order to demonstrate how this theoretical model works.