  The study of the shape and sub-structure of high p_T jets produced in hadron
collisions is becoming an increasingly important component of LHC phenomenology
in the context of new particle discoveries. We study here the state of the art
for masses of QCD jets in perturbation theory to all orders, up to
next-to--leading logarithmic accuracy, with inclusion of non-global logarithms.
We also study the precise role of the jet algorithm used to construct the jets
and its impact on resummed predictions. Such analytical predictions, where
available, offer an alternative to leading logarithmic predictions from Monte
Carlo event generators and could, after fixed-order matching and accounting for
non-perturbative effects, be used for direct comparison to early LHC data.
