The cold plasma environment of the Earthʹs magnetosphere supports the shear Alfven and the fast MHD wave modes. Both these modes encounter natural boundaries in the magnetosphere and can form resonant structures that have the potential of providing routine magnetospheric plasma diagnostic capabilities. The shear Alfven mode has magnetic field guided energy which forms standing field line resonances (FLRs) between conjugate ionospheres while the boundaries for fast mode waves are still unclear although the fast mode turning point as a boundary appears to be undisputed. Excitation mechanism candidates for ULF waves in the magnetosphere must explain the observation of the existence of FLRs almost all the time and at any place (down to L∼1.3) in the dayside magnetosphere. The entry of fast mode wave energy from the bow shock through the magnetopause and down to low latitudes may provide a partial answer. Perhaps FLRs are energised by fast mode resonant structures (cavity/waveguide modes) in addition to “general ULF activity” or “noise” in the system. Despite uncertainties in understanding the excitation of FLRs throughout the magnetosphere, they can be used to estimate the equatorial mass density on high latitudes field lines. Within the plasmasphere, the detection of consecutive FLR harmonics allows the plasma mass density to be determined at discrete, field aligned altitudes without assuming, apriori any density model. This paper reviews current understanding of ULF band, resonant structures in the magnetosphere, emphasizes the ubiquitous nature of FLRs, discusses the structure of fast mode wave resonances and their role in the excitation of FLRs, highlights the diagnostic possibilities available from both the FLR and fast mode resonance structures and identifies some directions for further work.