Recent investigations of two-dimensional airfoil stalling characteristics have revealed low-frequency and highly unsteady flow in some cases and large-scale three-dimensional structures in other cases. The latter were referred to as "stall cells" and may form on two-dimensional configurations where the ends of the airfoil model are flush with tunnel side walls or end plates. This paper presents results of detailed investigations of the stalling characteristics of several airfoils that exhibited both low-frequency unsteadiness and large-scale three-dimensional structures. The airfoils were wind-tunnel tested in a two-dimensional configuration. The primary measurements were spanwise wake velocity and mini-tuft flow visualization. The results showed that airfoils with trailing-edge separations at and above maximum lift (static stall) exhibited stall-cell patterns. Conversely, airfoils that had leading-edge separation bubbles that grew in size as the angle of attack was increased into stall developed the low-frequency, highly unsteady flow. This unsteadiness was found to be essentially two dimensional. Therefore, the development of either of these phenomena appears to be determined by the characteristics of the boundary-layer separation leading up to the stall.