One way of classifying epithelial tissue is by the number of layers it consists of. 

1. A simple epitheliumconsists exclusively of a single layer of cells. 
2. Stratified epitheliumis composed of 2 or more layers of cells. 
3. Pseudostratified epitheliumconsists of tightly packed cells which exist in one layer of cells, although they are different sizes. 
4. Simple squamous epitheliummainly lines blood vessels and body cavities, and permits the transport of substances between tissues. 
5. Simple cuboidal epitheliumis primarily located in glandular tissues and kidney tubules and assists in the absorption and transport of filtered substances. 
6. Simple columnar epithelial cellstypically line the stomach and intestines, and assist in absorption and secretion. These cells are assembled in a row with nuclei present at the same level in the tissue. 
7. Stratified squamous epithelial cellshave protective traits. 
8. Stratified cuboidal epithelial cellsare located in the excretory ducts of the sweat and salivary glands. Its primary function is structural support for the cell. Stratified columnar epithelial cells are found in the mucous membrane lining the eyelids. Its main functions are protection and mucous secretion. 
9. Pseudostratfied columnar epithelial cellsline the upper respiratory tract and typically consist of many cilia. 

Epithelial tissue can also be classified by specialized functions they possess. 

1. Transitional epitheliumlines the urinary tract, and allows the bladder to expand. It is able to do this as the multiple layers of cells are flattened when they are stretched. 
2. Olfactory epitheliumcontains the olfactory receptor cells, which transmits information from receptors in the nasal cavity to the brain (to interpret the smell). 
3. Glandular epitheliumis responsible for generating and secreting hormones, proteins, and water and is found in the glands.