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Several single dish surveys toward ultracompact HII regions have shown that usually hot, dense molecular gas is closely associated with those objects. Based on these surveys several regions have been selected for more detailed, interferometric studies using the VLA, Owens-Valley and the Plateau de Bure mm-arrays. Our VLA study of 4 objects in the NH$_3$(4,4) line shows that the regions traced by this line are molecular clump of size $\sim 0.1\,$pc, densities of $\sim 10^{7} \,$cm$^{-3}$ and temperatures of $100\,$K or higher. The most likely interpretation for these objects are young stellar objects which are still highly embedded in the molecular gas from which they formed. We also present the results of a high resolution survey toward 20 ultracompact HII regions in the $22\,$GHz water maser line which shows that strong maser emission is usually found to be located in front of the ionized arc in the case of the cometary class of ultracompact HII regions. Also, in all cases where high resolution ammonia maps exist, the water masers are coincident with the hot, dense molecular clumps. Finally, we present results of recent high resolution observations in the CH$_3$CN(6-5) and C$^{18}$O)1-0) lines toward 2 of the objects studied in the ammonia and water lines. In both cases we detect strong $3\,$mm continuum emission as well as high velocity gas coincident with the molecular clumps imaged in NH$_3$(4,4).
