Hot molecular cores are a short-lived phase of massive star formation: the massive stars have heated up the dense molecular gas in which they are deeply embedded, but have not yet ionized and disrupted it. Information on their structure (density, temperature, velocity, molecular abundances) is needed both for research on massive star formation and for astrochemistry. We constrain their structure by radiative transfer modeling of a variety of molecular lines, including from highly excited states, using APEX and Herschel/HIFI. To spatially resolve the hot molecular gas, we performed VLA and SMA observations. In this article, we summarize some to the results.