  Despite the fact that jets from black holes were first understood to exist
over 40 years ago, we are still in ignorance about many primary aspects of
these systems -- including the radiation mechanism at high energies, the
particle makeup of the jets, and how particles are accelerated, possibly to
energies as high as 100 TeV and hundreds of kpc from the central engine. We
focus in particular on the discovery (and mystery) of strong X-ray emission
from radio jets on kpc-scales, enabled by the unequaled high resolution of the
\emph{Chandra} X-ray observatory. We review the main evidence for and against
the viable models to explain this X-ray emission over the last 20 years.
Finally, we present results of a recent study on the X-ray variability of
kpc-scale jets, where we find evidence that between 30-100\% of the X-ray jet
population is variable at the tens-of-percent level. The short ($\sim$years)
variability timescale is incompatible with the IC/CMB model for the X-rays and
implies extremely small structures embedded within the kpc-scale jet, and thus
requires a reconsideration of many assumptions about jet structure and
dynamics.
