Electroweak phase transition (EWPT), as the phase transition from an electroweak symmetric phase to the electroweak broken phase in the early universe, is a cross over in the Standard Model (SM). However, the EWPT being strongly first order in nature is a necessary condition for electroweak baryogenesis, one of the most appealing mechanisms explaining the matter antimatter asymmetry mystery of par-ticle physics. On the other hand, the fast developing gravitational wave experiments may help us to understand the nature of the EWPT as strongly first order cosmological phase transitions can produce detectable gravitational wave signals. We investigate different ways to extend the SM Higgs sector such that the EWPT is strongly first order in nature, as well as how the extended Higgs sector is compatible with various experimental probes. We first explored the possibility of extending the Higgs sector with a real singlet which acquires a non-zero vacuum expectation value through a spontaneous Z2 breaking. In order for the EWPT to be strongly first order, such an extension requires a light singlet that mixes with the SM Higgs, which makes it visible to Higgs precision measurements and Higgs exotic decays. Another extension we explored is the well motivated Next-to-Minimal Supersymmetric Standard Model where the extended Higgs sector includes two Higgs doublets and a singlet. We pointed out there that the widely used criteria for a strongly first order phase transition based on the vacuum structure calculation can provide a misleading picture of the phase transitions, compared to that obtained from computing the tunneling probability for bubble nucleation. Such a feature may present in generic models with multiple dynamical fields as well as large tree level barriers.