This thesis deals with Higgs physics and related issues in the Standard Model (SM) and beyond. In the first part, theoretical predictions for Higgs pair production in the SM are given. The second part covers supersymmetry. In the framework of the MSSM, decays of a light squark are computed. Higgs mass corrections are calculated in the NMSSM. In the third part, phenomenological aspects of fermionic resonances in Composite Higgs Models are investigated.