A vast literature on the theory and phenomenology of two-Higgs-doublet models (2HDM) exists since long. However, the present situation demands a revisit of some 2HDM properties. Now that a 125 GeV scalar resonance has been discovered at the LHC, with its couplings to other particles showing increasing affinity to the Standard Model Higgs-like behaviour, the 2HDM parameter space is more squeezed than ever. We briefly review the different parametrizations of the 2HDM potential and discuss the constraints on the parameter spacearising from the unitarity and stability of the potential together with constraints from the oblique electroweak $T$ -parameter. We also differentiate the consequences of imposing a global continuous $U(1)$ symmetry on thepotential from a discrete $Z_2$ symmetry.