在电磁学里，磁石、磁铁、电流及含时电场，都会产生磁场。处于磁场中的磁性物质或电流，会因为磁场的作用而感受到磁力，因而显示出磁场的存在。磁场是一种矢量场；磁场在空间里的任意位置都具有方向和数值大小。

磁铁与磁铁之间，通过各自产生的磁场，互相施加作用力和力矩于对方。运动中的电荷会产生磁场。磁性物质产生的磁场可以用电荷运动模型来解释。

当施加外磁场于物质时，磁性物质的内部会被磁化，会出现很多微小的磁偶极子。磁化强度估量物质被磁化的程度。知道磁性物质的磁化强度，就可以计算出磁性物质本身产生的磁场。产生磁场需要输入能量，当磁场被湮灭时，这能量可以再回收利用，因此，这能量被视为保存于磁场。

A magnetic field is the magnetic effect of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. The term is used for two distinct but closely related fields denoted by the symbols B and H, where H is measured in units of amperes per meter (symbol: A·m or A/m) in the SI. B is measured in teslas (symbol:T) and newtons per meter per ampere (symbol: N·m·A or N/(m·A)) in the SI. B is most commonly defined in terms of the Lorentz force it exerts on moving electric charges.