The doctoral dissertation updates the effectiveness of internal communication in public companies in Croatia and defines the variables that need to be paid attention to when organizing internal communication. The fundamental scientific goal is to create an optimal model of adequate internal communication in public companies in Croatia.. With this intention, the main hypothesis was set that internal communication significantly affects the satisfaction of employees in a public company (H1) and three auxiliary hypotheses about the existence of a positive relationship between satisfaction with corporate information and communication climate (H2), a positive relationship between satisfaction with feedback and communication with superiors (H3), and the existence difference in satisfaction with horizontal communication depending on age and length of service (H4). The doctoral dissertation consists of two main parts, which include theoretical elaboration and research. The theoretical elaboration analyzes the available literature of foreign and domestic authors from the researched areas, while the research is conducted using the comparative method and qualitative methodology in three steps. First, a survey was conducted in three large public companies on a sample of 1,342 employees, in which the general satisfaction of employees with a focus on internal communication was examined. Then, using a comparative method, the similarities and differences between two qualitative surveys of the Croatian Post in which employee satisfaction was examined ten years apart were compared. Finally, the key persons responsible for internal communication, human resources and sales in public companies that participated in the implementation of the research were examined using a qualitative methodology, that is, an in-depth interview method. In a scientific sense, the doctoral dissertation proves that employee satisfaction with internal communication is associated with overall job satisfaction significantly more than sociodemographic characteristics are associated with job satisfaction. Particularly important aspects of internal communication were satisfaction with the communication climate and satisfaction with communication with superiors. The results of the research suggest that public companies should continuously invest in the development and improvement of internal communication processes in order to increase employee satisfaction in their workplaces, which can be achieved by ensuring transparency, encouraging two-way communication, regularly informing employees and promoting involvement and teamwork through communication channels within the company. The results of this doctoral dissertation can help public companies provide adequate resources for investing in optimal internal communication in order to positively influence general job satisfaction and employee engagement