It is customary to represent the magnetic field of the Earth by a series of spherical harmonic coefficients of the magnetic potential up to a certain degree and order, usually about thirteen. Although this has certain computational advantages, such as upward and downward continuation, we show that there are also serious disadvantages. For instance, the use of spherical harmonics up to a certain degree does not remove from the surface fields all wavelengths greater than a certain value. This can be shown both theoretically and using observations. It is also shown that the total field magnitudes represented by spherical harmonics up to a certain degree have minimum wavelengths equal to the circumference of the Earth divided by twice the maximum degree of harmonic used. We discuss briefly the implications concerning how surface fields are separated into core and crustal components.