This chapter introduces Geographically and Temporally Weighted Regression (GTWR), an extension of Geographically Weighted Regression (GWR) that incorporates temporal variations into spatial analysis. Also referred to as Spatially and Temporally Geographically Weighted Regression (STGWR), this model enhances analytical insights by accounting for both spatial and temporal dependencies. The core element of GTWR is the spatio-temporal distance metric, which determines data inclusion in individual regressions. We estimate GTWR model in R software to examine demographic changes in Poland from 2018 to 2021, analyzing spatial and temporal patterns in fertility levels and the influence of population structure and economic factors. This chapter equips researchers with tools to uncover complex spatio-temporal relationships, improving their understanding of dynamic interplay between space and time.