This paper addresses experimental approaches using spectroscopic diagnostics, and theoretical approaches using mathematical modeling, to visualize electromagnetic-thermal-fluid phenomena in arc welding. Arc welding is a complicated system in which four states, solid, liquid, gas, and plasma, coexist within a space of about 1 cm3. Spectroscopic measurements of line spectra from arc plasma allow us to obtain the plasma temperature. An intentional lighting and filtering technique visualizes molten metal behaviors during arc welding. Numerical simulation based on a particle method reveals flows conveying heat energy in a droplet and a weld pool. A numerical method with high accuracy makes it possible to simulate the fluid dynamic behavior of arc plasma and transport of nanoparticles growing in the peripheral region of the arc plasma. This paper also presents a discussion of the results obtained using these approaches, especially from a fluid mechanics standpoint.