In this chapter you will learn what epithelia are, what their main characteristics are, how they are classified, what their embryonic origin is and what their functions are. You will learn how cells organise themselves to form epithelia, and that epithelia and their cells are polarised structures. You will also learn that epithelial cells are constantly renewed. Finally, you will learn that some epithelia (glandular epithelia) are specialised in the production of substances that they secrete onto surfaces or free spaces or into other neighbouring tissues. You will see what types of glandular epithelia exist, how their secretion mechanisms are and how these epithelia originate in the embryo.