During the past two decades intensive experimental and theoretical effort has progressed our knowledge and understanding of high temperature superconductivity. Despite this progress, our understanding of important aspects is as yet incomplete. A complete understanding of this phenomenon is very promising as it may lead to "room temperature superconductivity" that will revolutionize the energy and communication industries together with many other gifts for technology. In this thesis, an effort is made to characterize these materials using optical spectroscopy. A new method for calibration of optical reflectivity spectra is presented, which increases considerably the accuracy and reproducibility of the spectra. Two different families of high temperature superconductors are studied and some unique properties of each family are revealed. The newly-discovered iron pnictide family with more than one Fermi surface and a magneto-structural phase transition shows interesting features in optical spectra. A doping-dependent study of this family reveals signs for quantum criticality. In the family of mercury based cuprate high Tc superconductors, the role of interactions is studied by comparing the experimental data with Migdal-Eliashberg theory of strong coupling. In addition, the energy dependent relaxation rate and normalized mass uncover the existence of a Fermi liquid in the pseudo-gap phase of this compound. Below the pseudogap temperature, a universal collapse of the frequency and temperature dependent relaxation rate on a single curve supports the Fermi liquid picture, albeit with a coefficient relating temperature and frequency dependence that is slightly different from the theoretical prediction for a Fermi liquid. Furthermore, a new method for calculation of optical spectra from the Migdal-Eliashberg theory is introduced that causes up to an order of magnitude increase in calculation speed. Inspired by the Fermi liquid picture and based on the Migdal-Eliasherg theory, a minimal model is introduced which explains aforementioned frequency and temperature dependence of the optical spectra of the mercury-based cuprate high Tc compound.