  This letter is part of a two-letter tutorial on radio localization and
sensing, with focus on mobile radio systems in 5G mmWave and beyond. Part I
introduces the fundamentals, covering an overview of the relevant literature,
as well as the different aspects of localization and sensing problems. Then,
different performance metrics are presented, which are important in the
evaluation of methods. Methods are detailed in the last part of this letter.
Part I thus provides the necessary background to delve into more
forward-looking problems in Part II.
