We are not able, at least for the moment, to define in a rigorous mathematical fashion a meaningful interacting QFT in $3+1$ dimensions that is coherent with the perturbative theory utilized by physicists (in more precise words, that satisfies the Wightman axioms).
On the contrary, some rigorous interacting QFTs can be defined in lower (spatial) dimensions. Take for example the $\phi^4$ model: it is not well-defined in $3+1$ dimensions, but it is in $1+1$ and $2+1$ (the latter for sure on finite volume, I am not sure about the infinite volume limit) as proved by Glimm and Jaffe in the sixties. This simplified models, even if they may not be physically interesting, has been analyzed in the hope that the same tools may be utilized in the meaningful theories. Unluckily, this has not been the case so far (anyways, there is still ongoing work on the subject, especially concerning the Yang-Mills theory).
However there is no rigorous "no-go theorem" that says that the interacting quantum field theories in $3+1$ dimensions cannot be constructed, but it may be possible that for some model the limitations are more fundamental and not only related to the lack of mathematical tools (see the comment by A.A. below).
This post imported from StackExchange Physics at 2015-05-31 13:10 (UTC), posted by SE-user yuggib