Since the early ages of the turbofan revolution, in the early 1960's, fan noise has been a challenge for noise engineering in aircraft design. Given its tonal acoustic signature, which particularly is annoying to human beings, this noise source has been penalized in aircraft certification. Acoustic Liners are passive devices installed at the internal wall of turbofan nacelles to mitigate fan noise. Although being successfully applied for decades, the trend of larger fans, with lower rotational speeds and shorter nacelle, motivated by the seek for higher fuel efficiency, has brought a new challenge for liner design. In this talk, I will present the key aspects related to acoustic liners modelling for turbofan application. Special attention will be given to the boundary conditions used to model these structures. The experimental methods used to characterize under realistic turbofan conditions will also be discussed, followed by the upcoming challenges in novel liner design.