Membrane proteins (MPs) fulfill essential roles in trans-membrane material transport, energy conversion, information transduction, and the maintenance of membrane homeostasis. What are the physical factors that differentiate MP molecules from water-soluble proteins? What are the typical structural characteristics of MP molecules? How are they integrated into the biomembrane? This chapter will explore the significance of MPs and their classification. To briefly summarize, there are two primary classes of MP molecules: α-helix bundles and β-barrels. The majority of functional MPs belong to the α-helix bundle category, which will be extensively discussed in subsequent chapters. In this chapter, we will dedicate a section to examine the folding process and stability of MP molecules, using the β-barrel type MP as an illustrative example.