In metrology, fundamental quantities are the quantities on which the measurements of all other quantities depend and to which all other quantities can be related. They appear also in the physical theories on which the units are based. The set of fundamental quantities forms the basis of any system of units.

The definitions and quantities of fundamental quantities are laid down by the International System of Quantities (ISQ). The ISQ is a universal set of quantities recognized by the International Committee for Weights and Measures (CIPM) and the International Organization for Standardization (ISO), from which all other quantities used in science, technology, and everyday life can be derived.

Seven fundamental quantities are listed in the ISQ, each with its own base unit and symbol. These quantities and their corresponding units are:

1. Length (meter, m)

2. Mass (kilogram, kg)

3. Time (second, s)

4. Electric current (ampere, A)

5. Thermodynamic temperature (kelvin, K)

6. Amount of substance (mole, mol)

7. Luminous intensity (candela, cd)

These fundamental quantities form the foundation of the International System of Units (SI), which is the most widely used system of units in the world. The SI units for these fundamental quantities are derived from fundamental physical constants and natural phenomena, such as the speed of light, the Planck constant, and the freezing point of water.