Talk abstract: Solar and stellar flares are caused by catastrophic release of magnetic energy stored in the corona, which results in plasma heating, particle acceleration and intense radiation across the electromagnetic spectrum. Multiwavelength observations are highly important for understanding the nature of flares, because different flare-related physical processes reveal themselves in different spectral ranges. While the Sun is routinely monitored by many instruments at various wavelengths, multiwavelength observations of stellar flares are far less common. To study the correlation between thermal and nonthermal processes in stellar flares, we searched the databases of Kepler (optical observations) and XMM-Newton (soft X-rays) for the flares observed simultaneously with both instruments; eight distinctive flares (with the energies exceeding 1032 erg) on three stars (of K-M spectral classes) have been found. We analyze and compare the flare parameters (released energy and rise and decay times) in the optical and X-ray spectral ranges; we also analyze the electron spectra inferred from the X-ray observations. On average, stellar superflares look like scaled-up versions of solar flares, which indicates the same underlying mechanism. At the same time, like in the solar flares, there are significant variations from flare to flare, including variations of the thermal-to-nonthermal emission ratio.