In this chapter, a theoretical model for understanding the knowledge of mathematics teachers for teaching modelling in digital environments (TPMMK) is outlined and discussed. This model acknowledges the multifaceted nature of knowledge required to address contemporary educational demands, highlighting the importance of extra-mathematical knowledge in the teacher training. To achieve this, arguments are presented based on literature reviews. Drawing on data from exploratory case study regarding the modelling tasks designed by future mathematics teachers the potential of this theoretical proposal to describe teachers’ knowledge related to extra-mathematical, mathematical, technological, and pedagogical aspects and their interactions is exemplified.