In the first part of this thesis we introduce the Two Higgs Doublet Model (2HDM) with a complex singlet and a Z 3 Symmetry (2HDMS). We derive a physical input basis and discuss theoretical constraints. These include unitarity constraints, boundedness from below and the stability of the electroweak vacuum. Furthermore we use experimental constraints from flavor physics, Higgs boson searches and measurements of the Standard Model Higgs boson to constrain the parameter space in this model. In this model we discuss a 3σ signal reported by CMS and LEP and interpret it as a light Higgs boson with a mass of 95 GeV. For this singlet-like state we evaluate the experimental coupling uncertainties at a future linear collider. Following that, we compare this to the similar N2HDM which is the extension of the 2HDM with a real singlet. We make first efforts to exploit experimental methods for distinguishing both models. We find that the 2HDMS and the N2HDM can both accomodate the excesses at LEP and CMS simultaneously and are difficult to distinguish using the evaluation of coupling uncertainties at a linear collider. Further possibilities to distinguish both models using the properties of the CP-odd Higgs bosons are discussed. In the second part of the thesis I explore the impact of a recent analysis published by CMS with searches for heavy Higgs bosons in final states with up to four top quarks. The original Beyond the Standard Model interpretation of the analysis is limited to strictly aligned CP-even and CP-odd Higgs bosons and to masses between 350 and 650 GeV. Furthermore it places strong constraints on the important quantity tanβ. In the 2HDM low values of this quantity are often needed to describe scenarios with baryogensis. We perform a recasting of this analysis with the Monte-Carlo- Event generator Madgraph5 and the code Madanalysis to extend the analysis for more general types of models, such as models with CP-violation and higher masses of up to 1 TeV. We obtain efficiencies in the most sensitive signal region and derive fit functions as a function of the mass. These fit functions are used to derive the efficiency as a function of the CP-even, CP-off top Yukawa and gauge-boson couplings. We implement our results in the code Higgsbounds, which can test models with multiple scalars against a large number of Higgs boson searches from LEP, CMS and Atlas. We then use this implementation to study the impact of the four-top analysis on the low tanβ region in the 2HDM, its singlet extensions and the complex 2HDM (C2HDM).