Giant planets are planetary bodies that primarily consist of hydrogen and helium, and a small fraction of heavy elements (i.e., rocks, ices). Since giant planets affect the dynamics of young planetary systems, and their compositions provide information on the birth environment of planets, a key objective in planetary science is to understand how these planets form and what they are made of. First, I will summarize the two giant planet formation models: core accretion, the standard model, and disk instability. There are substantial differences between these two formation models, including formation timescale, favorable formation location, ideal disk properties for formation, and early evolution. In addition, the two models make different predictions about the bulk compositions and structures of the planets. It is therefore required to constrain the compositions and internal structures of giant planets in the solar-system and around other stars. Next, I will discuss how planetary interiors are modeled, and present updated interior models of solar-system the outer planets. Finally, I will discuss why combining research of solar and extrasolar planets, and synthesizing theory and observations can enhance our knowledge of giant planets.