The units used to measure the fundamental quantities are called fundamental units or basic units; that is the fundamental units are the units of length, mass, time, electric current, temperature, luminous intensity and amount of substance. Scientists gather information with their senses like eyes, ears, etc. and make observations. Some observations are simple like figuring out the texture and colour while other observations may be complex for which they may need to take measurements. Measurement is one of the fundamental concepts in science. Without the ability to measure, a scientist wouldn’t be able to gather information and form a theory or conduct experiments. In this chapter, the units of physical quantities and methods of evaluating them are discussed, while the other section of the chapter deals with the errors that can occur while taking measurement and significant figures.