In this talk I will discuss some theoretical subtleties of the top-quark mass determination, issue which persists in being highly controversial.

In the first part of the talk I will compare several NLO generators for top-pair production implemented in the POWHEG BOX framework, that differ by the level of accuracy employed to describe the top decay. The impact of the shower Monte Carlo programs, used to complet the NLO events generated by POWHEG BOX, is also studied. In particular, I discuss the two most widely used shower Monte Carlo programs, i.e. Pythia 8.2 and Herwig 7.1, and how it is possible to interface them with processes that contain decayed emitting resonances. The second part is instead devoted to the study of linear renormalons in observables that can be employed to determine the top mass.