Auroral Medium Frequency Burst (MFB) is the least understood of three types of terrestrial auroral emissions detectable at ground level. MFB consists of broadband (500-2000 kHz) left-polarized impulsive emissions typically occurring for a few minutes at the onset of polar substorms, one of the most energetic phenomena in the terrestrial magnetosphere. Recent observations of the source location and fine structure of MFB provide the best opportunity yet to test theoretical models of the generation mechanism. Proposed mechanisms include mode conversion of Langmuir or electron cyclotron sound waves excited via resonant interactions with auroral electrons; the former has been shown under certain conditions to predict the frequency-time characteristics of MFB fine structure.