  Hardware-based track reconstruction in the CMS and ATLAS trigger systems for
the High-Luminosity LHC upgrade will provide unique capabilities. An overview
is presented of earlier track trigger systems at hadron colliders, in
particular for the Tevatron CDF and D0 experiments. We discuss the plans of the
CMS and ATLAS experiments to implement hardware-based track reconstruction for
the High-Luminosity LHC. Particular focus is placed on the track trigger
capability of the upgraded CMS experiment. We discuss the challenges and
opportunities of this novel handle, and review the alternatives that were
considered for its implementation as well as discuss the expected performance.
The planned track trigger systems for CMS and ATLAS have different goals, and
we compare and contrast the two approaches.
