In the past decade, impulsive hard X-ray bursts have been extensively observed (cf. Kane et al., 1980). These observations have increased our knowledge of the energy spectrum of the accelerated electrons and their temporal evolution. However, because of the lack of spatial resolution and direct plasma diagnostics, many important questions concerning the nature of the impulsive phase are still left unanswered. Since direct imaging of hard X-rays above 30 keV with high resolution of is still beyond our present technology, we have to use other indirect means to deduce the spatial structure of the hard X-ray source. With the recent launch of the Solar Maximum Mission (SMM) satellite, we are able to obtain correlated observations of the flare impulsive phase in hard X-ray and simultaneously in the UV lines of Si IV (1402 Å) and 0 IV (1401 Å). The Si IV/0 IV intensity ratio is density sensitive and therefore provides plasma diagnostics in the emission region. Analysis of the spatially resolved UV observations with the correlated hard X-ray observations allows us to study the spatial structure and physical conditions in the UV and hard X-ray sources (Cheng et al., 1981; 1982; 1984). Descriptions of the various solar instruments on SMM can be found in Solar Physics (vol. 65, pp 5-116). In this paper, I briefly summarize the important observational results and discuss their theoretical interpretation.