Solar flare precursors

Recent progress in the study of flare precursors and onset is reviewed. New theoretical results on filament formation and erution are presented, including especially the dynamical effects of magnetic fields. Ample evidence is found that energetic processes are already at work in the onset phase, a few minutes before the rapid rise of the hard-radiation impulse. In particular, the prevalence of soft and hard X-ray preheating (to less than 100 million K) is detailed, along with a connection to the launch of coronal mass ejections. A possible interpretation of the empirical time profiles near onset is that 'preheating' signifies that the flare has slowly started, and the transition to the impulsive stage then represents a change of phase in the flare-instability process. Finally, the problem of the interpretation of microwave preflare data is delineated.