  4-dimensional (4D) trackers with ultra fast timing (10-30 ps) and very fine
spatial resolution (O(few $\mu$m)) represent a new avenue in the development of
silicon trackers, enabling new physics capabilities beyond the reach of the
existing tracking detectors. This paper reviews the impact of integrating 4D
tracking capabilities on several physics benchmarks both in potential upgrades
of the HL-LHC experiments and in several detectors at future colliders, and
summarizes the currently available sensor technologies as well as electronics,
along with their limitations and directions for R$\&$D.
