Growth of polycrystalline silicon from fluidized bed reactors (FBR) produces two general types of silicon products: granular material (diameters on the order of mm) and homogeneously nucleated material often called nanopowder (diameters in the range 10-100 nm). Nanopowder particles tend to be amorphous and have a spherical morphology with an average particle diameter of similar to 80 nm. Granular material is generally spherical, highly twinned, polycrystalline with crystallite sizes that can reach 200 nm, and includes regions of porosity. The porosity is similar to 1-4 volume percent, and only the smallest pores exhibit evidence of amorphous regions along the pore surface. The amount of nanopowder that agglomerates on the granular material has been identified using transmission electron microscopy, but agglomeration plays only a minor role in the overall growth process. Therefore, it is proposed that the primary mechanism for granular formation in commercial FBR is chemical vapor deposition, and the pores are associated with nanopowder agglomeration and incomplete sintering.