  Precise manipulation of individual charge carriers in nanoelectronic circuits
underpins practical applications of their most basic quantum property --- the
universality and invariance of the elementary charge. A charge pump generates a
net current from periodic external modulation of parameters controlling a
nanostructure connected to source and drain leads; in the regime of quantized
pumping the current varies in steps of $q_e f$ as function of control
parameters, where $q_e$ is the electron charge and $f$ is the frequency of
modulation. In recent years, robust and accurate quantized charge pumps have
been developed based on semiconductor quantum dots with tunable tunnel
barriers. These devices allow modulation of charge exchange rates between the
dot and the leads over many orders of magnitude and enable trapping of a
precise number of electrons far away from equilibrium with the leads. The
corresponding non-adiabatic pumping protocols focus on understanding of
separate parts of the pumping cycle associated with charge loading, capture and
release. In this report we review realizations, models and metrology
applications of quantized charge pumps based on tunable-barrier quantum dots.
