  We show that the Tensor Renormalization Group (TRG) method can be applied to
O(N) spin models, principal chiral models and pure gauge theories (Z2, U(1) and
SU(2)) on (hyper) cubic lattices. We explain that contrarily to some common
belief, it is very difficult to write compact formulas expressing the
blockspinning of lattice models. We show that in contrast to other approaches,
the TRG formulation allows us to write exact blocking formulas with numerically
controllable truncations. The basic reason is that the TRG blocking separates
neatly the degrees of freedom inside the block and which are integrated over,
from those kept to communicate with the neighboring blocks. We argue that the
TRG is a method that can handle large volumes, which is crucial to approach
quasi-conformal systems. The method can also get rid of some sign problems. We
discuss recent results regarding the critical properties of the 2D O(2)
nonlinear sigma model with complex beta and chemical potential. As some of
these results appeared in a recently published paper (PRD 88, 056005) and two
recent preprints (arXiv:1309.4963 and arXiv:1309.6623), these proceedings
rather emphasize the conceptual aspects of our ongoing effort.
