There is a wealth of experimental data indicating very similar patterns for emitted particles in small collision systems from p+p to p+A to d/3He+A as compared with large collision systems (A+A) at both RHIC and the LHC. In this talk we provide a critical assessment of various theoretical approaches to understand the basic physics at play leading to these experimentally observed phenomena. These theoretical approaches include QGP formation and hydrodynamic evolution, particle transport with well defined quasiparticles, and initial state correlations via saturated gluonic matter.