Membranes are an integral part of our life and used for various aspects of our life, such as respiration, drinking water, tea making, juice preparation, and air conditioners. Their low cost, effectiveness, and efficient separation properties make them a favorite of every industry. This chapter explains the basics of membrane science. It covers all about membranes from their classification, categorization, advantages, disadvantages, and history. It is explained in this chapter how various membrane properties, based upon different driving forces, are used to categorize the membranes, with examples included. Also, the developmental history of membranes is presented in a chronological order along with their advantages and disadvantages. Last, as it is important for every process to be successful, it should have some applicability to various industrial processes and day-to-day life activities so as to make life more comfortable and easy. Therefore, important fields employing membranes for various applications are provided at the end of the chapter.