Physics is the science of measuring nature in a precise way. Some quantities like length, mass, time, temperature, current, light intensity and amount of substance are called fundamental quantities, and each has a standard unit such as metre, kilogram, second, kelvin, ampere, candela and mole. From these, other quantities like speed, force and energy are built, and their units are called derived units (e.g., newton, joule, watt). To avoid confusion, the whole world follows the International System of Units (SI) with fixed symbols like m, kg, s, A, J and N. Because measurements can be huge or tiny, we use scientific notation (powers of 10) and follow rules of significant figures to show accuracy. Another key tool is dimensional analysis, which helps check if equations make sense and to find relations between quantities. This chapter is the foundation of physics—it teaches the common language of measurement, precision and correctness.