This chapter examines the theoretical underpinnings as well as the technological and economic attributes of High-Speed Rail (HSR). It additionally delineates the regional economic ramifications of HSR from a spatio-temporal viewpoint, focusing specifically on two principal dimensions: alterations in spatio-temporal perceptions and the spatio-temporal variability of its economic repercussions. This chapter examines the mechanisms that account for geographical and temporal differences in these impacts, utilizing two primary analytical frameworks: agglomeration externalities and network externalities. Each approach provides significant insights into how HSR transforms regional economic landscapes, while also adding intricate dynamics that fluctuate across both spatial and temporal dimensions.