The visual observations of batch and continuous foaming processes were conducted to understand the bubble nucleation and bubble growth behaviors in polymers. The batch foaming was performed using a newly developed high-pressure cell, where two sapphire windows were equipped on the walls so as to observe the early stage of bubble nucleation and growth behaviors with the help of a high-speed digital camera and microscope. In the batch process, homo polypropylene was foamed at different pressure release rates using CO₂ as a physical blowing agent to see the effects of operating condition on the bubble nucleation and growth rates. The in situ observation could identify that (1) the bubble nucleation and growth occur simultaneously, (2) the influence region, where the nucleation was suppressed, exists around bubbles, and (3) the nucleation rate and the growth rate increase as the pressure release rate increases. The continuous foaming was performed using a different visualization unit, which consists of an autoclave and an extrusion slit-die with quartz windows. It was found that the nucleation mechanism in the continuous foaming, i.e., extrusion foaming, was different from that of batch foaming. In the continuous extrusion foaming, the nucleation could be induced by flow and/or shear stress, and by cavitations brought by the surface roughness of the wall.