Abstract. This paper, a review of the present status of existing models for particle acceleration during impulsive solar flares, was inspired by a week-long workshop held in the Fall of 1993 at NASA Goddard Space Flight Center. Recent observations from Yohkoh and the Compton Gamma Ray Observatory, and a reanalysis of older observations from the Solar Maximum Mission, have led to important new results concerning the location, timing, and eificiency of particle acceleration in flares. These are summarized in the first part of the review. Particle acceleration processes are then discussed, with particular emphasis on new developments in stochastic acceleration by magnetohydrodynamic waves and direct electric field acceleration by both sub-and super-Dreicer electric fields. Finally, issues that arise when these mechanisms are incorporated into the large-scale flare structure are considered. Stochastic and super-Dreicer acceleration may occur either in a single large coronal reconnection site or at multiple "fragmented" energy release sites. Sub-Dreicer acceleration requires a highly filamented coronal current pattern. A particular issue that needs to be confronted by all theories is the apparent need for large magnetic field strengths in the flare energy release region.

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We present a wide-Ðeld, high dynamic range, high-resolution, long-wavelength (j \ 90 cm) VLA image of the Galactic center region. The image is centered on Sgr A, covers an area of 4¡ ] 5¡ with an angular resolution of 43A, and has a rms sensitivity of B5 mJy beam~1. The image was constructed from archival (1989 and 1991) VLA data of Pedlar et al. and Anantharamaiah et al. using new threedimensional image restoration techniques. These three-dimensional imaging techniques resolve the problem of non-coplanar baselines encountered at long wavelengths and yield distortion-free imaging of far-Ðeld sources with improved sensitivity. At j \ 90 cm the VLA is sensitive to both thermal and nonthermal emission and the resulting image gives an unprecedented contextual perspective of the largescale radio structure in this unique and complicated region. We have catalogued over a hundred sources from this image and present for each source its 90 cm Ñux density, position, and size. For many of the small-diameter sources, we also derive the 20/90 cm spectral index. The spectral index as a function of length along several of the isolated nonthermal Ðlaments has been estimated and found to be constant. We have found six new small-diameter sources, as well as several extended regions of emission, which are clearly distinct sources that have not been previously identiÐed at higher frequencies. These data are presented as a Ðrst epoch of VLA observations that can be used to search for source variability in conjunction with a second epoch of observations that were recently initiated.

We present a wide-field, subarcminute-resolution VLA image of the Galactic center region at 330 MHz. With a resolution of $7 00 ; 12 00 and an rms noise of 1.6 mJy beam À1 , this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency. The improved sensitivity has more than tripled the census of small-diameter sources in the region, has resulted in the detection of two new nonthermal filaments (NTFs), 18 NTF candidates, and 30 pulsar candidates, reveals previously known extended sources in greater detail, and has resulted in the first detection of Sagittarius A* in this frequency range.