This thesis systematically analyses the development of the water infrastructure between the twelfth and the sixteenth centuries in medieval Zagreb. Based on the primary sources of various types, e.g. litigations, last wills, and different types of decrees, it identifies key types of waterways that influenced the development of Gradec and Kaptol, two jurisdictions of which Zagreb consisted. Among the most crucial ones are the river Sava and the stream Medveščak which were used for transport, fishing, washing, and other activities, but also served as a driving force for numerous mills and bathing houses built on them. The thesis contributes to the fields of social, political and ecohistory. The main argument is that these fields cannot be looked at separately because the significance which control over water brought led to it being a cause of political turmoils. Mills and bathing houses are the best-documented water devices and despite different nature of their usage are one of the fundamental parts of this thesis. The conclusion is that mills were one of the driving forces for economic prosperity. On the other hand, the thesis shows that the bathing houses were a part of the culture, as a place of gathering and health services and as such, influenced social climate in the town. The advance of the water technology had long-term consequences on the transformation of the environment and fastened the process of urbanization.