This chapter discusses issues related to the effectiveness of internal communication from a praxeological point of view. In the light of quantitative and qualitative research results, the authors will analyse the categories of efficiency, quality, and the effectiveness of internal communication, along with their components. They will also present a theoretical model of interdependence between the determinants of internal communication and its effectiveness, as well as presenting a measuring instrument—an evaluation sheet for the effectiveness of internal communication, which gives rise to an integrated index of internal communication effectiveness. What is more, a list of factors that increase or decrease internal communication effectiveness will also be presented. On the basis of the conclusions, the model of internal communication will be modified and updated.