The quality of continuous-cast steel is greatly influenced by fluid flow in the mold, particularly at the meniscus. Recent examples of computational model applications at the University of Illinois are presented to investigate the formation of several different types of defects related to flow phenomena. The amount of gas injection into the tundish nozzle to avoid air aspiration is quantified by modeling. Computational model calculations of superheat transport and surface level fluctuations are presented. Meniscus defects, such as subsurface hooks and their associated inclusions, may form if the superheat contained in the steel is too low, or if top-surface level fluctuations are too large. A thermal stress model has been used to compute the distortion of the meniscus during a level fluctuation. Gas bubbles and inclusion particles may enter the mold with the steel flowing through the submerged nozzle. In addition, mold slag may be entrained from the top surface. These particles may be removed safely into the slag layer, or may become entrapped into the solidifying shell, to form sliver or blister defects in the rolled product. Transient, turbulent flow models have been applied to simulate the transport and entrapment of particles from both of these sources. The insights gained by these modeling efforts aid greatly in the development of processing conditions to avoid the formation of these defects.