ISO 26262 is an international standard valid for the automotive domain. It regulates all the activities to perform for developing safety critical systems in such domain. To be compliant with ISO 26262, all the required activities have to be performed and all the required work products have to be provided. Furthermore, in addition to develop a system in a safe way, following the safety standard guidelines, the achieved safety has also to be demonstrated. This is done through a safety case, a structured argument showing that a system is acceptably safe. ISO 26262 focuses on single systems and does not contain guidelines for product lines. Product line engineering is a valid approach to systematize reuse, aimed at reducing the effort needed to develop similar systems. But, it loses its strength when dealing with safety critical systems, since it is not aligned with safety standards. Hence, when developing a safety critical product line in the automotive domain, the work products required by ISO 26262 have to be provided every time from scratch, including the safety case, for each single system of the product line. This thesis work focuses on providing an approach for building and modeling a safety case for safety critical product lines in the automotive domain. Furthermore, the considered product line engineering approach is aligned with ISO 26262, through the inclusion of safety activities in the product line development process. Giving in this way, the concrete possibility to overtake to the current limitations, reducing the effort needed to develop and certificate each single system of a safety critical product line. To illustrate the validity of the proposed approach a safety critical product line developed by Scania is used as case study.