Geodesy is the science of studying the shape, size and gravity field of the Earth (International Association of Geodesy 2025) This involves measuring and representing the geometry, orientation and gravity field of the Earth, taking into account their time variability. Obviously, geodesy is the backbone for many applications where accurate geospatial information is needed, ranging from mapping and monitoring Earth processes and climate change impacts such as surface deformation, hydrology, tectonics, sea level rise and ice melt; time keeping; engineering and construction; vehicle navigation. This introductory chapter does not aim to elaborate on this, but instead showcase the work of Peter Teunissen; within the geodetic community known as the expert in mathematical geodesy, and the successor of two other famous geodesists from Delft, Prof. Jacob Menno Tienstra and Prof. Willem Baarda. They are acknowledged as the founders of the Delft School of mathematical geodesy (Molenaar 2021). In this chapter, we will first very briefly review the contributions of Tienstra and Baarda and then show how Peter Teunissen expanded the Delft school with a large focus on developing the theoretical framework for very precise estimation and quality control for positioning with Global Navigation Satellite Systems (GNSS).