Epithelial tissue consists of densely packed cells. It is either simple or stratified, based on the number of cell layers. Simple epithelium has one layer of cells, while stratified epithelium has multiple layers. Epithelial tissue may have squamous-, cuboidal-, or columnar-shaped cells. Squamous cells are flat, square cells. Cuboidal cells form a box or cube. Columnar cells are stacked, forming a column taller than they are wide. There are two surfaces to epithelial tissue: one side is firmly attached to the underlying structure, while the other forms the lining. The epithelium forms a barrier, allowing the passage of certain substances, while impeding the passage of other substances.