  We study the reconstruction of high p_T hadronically-decaying top quarks at
the LHC. The main challenge in identifying energetic top quarks is that the
decay products become increasingly collimated. This reduces the efficacy of
conventional methods that exploit the topology of the top decay chain. We focus
on the cases where the decay products of the top quark are reconstructed as a
single jet, a "top-jet". The most basic "top-tag" method based on jet mass
measurement is considered in detail. To analyze the top-tagging method,
theoretical and experimental aspects of the QCD jet background are examined.
Based on QCD factorization, we derive a simple analytic approximation for the
shape of the QCD jet mass spectrum. We observe a good agreement with the Monte
Carlo simulation. We consider high-p_T t\bar{t} production in the Standard
Model as an example, and show that our theoretical QCD jet mass distributions
can efficiently characterize the background via sideband analyses. We show that
with 25 fb^{-1} of data, our approach allows us to resolve top-jets with p_T
$\ge$ 1 TeV from the QCD background, and about 1.5 TeV top-jets with 100
fb^{-1}, without relying on b-tagging. To further improve the significance we
consider jet shapes (analyzed in 0807.0234 [hep-ph]), which resolve the
substructure of energy flow inside cone jets. A method of measuring top quark
polarization by using the b-quark p_T is also presented. The main advantages of
our approach are: (i) the mass distributions are driven by first principle
calculations, instead of relying solely on Monte Carlo simulation; (ii) for
high p_T jets (p_T $\ge$1 TeV), IR-safe jet shape variables are robust against
detector resolution effects. Our analysis is applicable to other boosted
massive particles such as the electroweak gauge bosons and the Higgs.
