Some materials become magnetized when placed in a magnetic field. The ability of a material to become magnetized is called magnetic permeability. An example of this is rubbing a piece of iron with a magnet. The iron will become magnetized and have its own magnetic field.

Many substances, even water, have a degree of magnetic permeability. When a material is placed in a magnetic field, it interacts with the field in one manner or another. The permeability of a substance describes the way the material responds and the effects of the field on the material. A substance with magnetic permeability will either magnetize itself in the direction of the field or in opposition to it. Thus, depending on the permeability, the substance will either be attracted or repelled by the field.

Scientists represent magnetic permeability with the Greek letter, mu (μ). The International System of Units (SI) measures permeability in henres per meter (H/m) or in newtons per ampere squared (N/A²). The air in a vacuum, also called free space, has a constant value which is called the magnetic constant. This value is represented by the symbol μ₀ and is valued as 4π×10⁻⁷ H/m which is about 1.2566×10⁻⁶ H/m. The numeric value is the same when measured in N/A².

Magnetic permeability is not a constant figure. Rather, it changes with the position of the material in relation to the magnetic field, the frequency of the field, humidity, temperature, and other factors. The permeability of a material can also be useful for various industries. Materials with high permeability are used in a variety of applications including in electromagnets, transformers, and inductors. Substances can also be characterized depending on their magnetic permeability.

If a substance is classified as paramagnetic its permeability is measured as a little more than one. Such substances are weakly attracted to magnets. If a substance is classified as diamagnetic, the permeability is measured as less than one. Such a substance will be repelled by a magnet. Further, a material can be classified as ferromagnetic.

Ferromagnetic metals have the greatest permeability of all substances and will become magnetized when exposed to a magnetic field. When the magnetic field the ferromagnetic substance is exposed to increases, the magnetic permeability will increase until it reaches its maximum value and then decrease. Some ferromagnetic materials will become hard or soft magnets. In materials that become soft magnets, such as those materials used in electromagnets, the material will lose its magnetism when the magnetic field is removed. Hard magnets are hard to magnetize, however, but will remain magnetized even when the magnetic field is removed.