We have investigated the physics of flares in M-dwarfs by means of optical/X-ray observations and modeling. The great efficiency of current optical spectrographs and detectors has allowed us to detect and analyze a great number of non white-light flares with intermediate spectral resolution and high temporal resolution. Although this kind of flares is the most typical on the Sun, few such events have been so far recorded on stars. We have obtained the physical parameters of the chromospheric flaring plasma (electron temperature, electron density, optical depth and temperature of the underlying source) by using a model that minimizes the difference between the observed Balmer decrements and the calculated ones, which result from solving the radiative transfer equation. On the other hand, the great sensitivity, wide energy range, high energy resolution and continuous time coverage of the EPIC detectors (on-board the XMM-Newton satellite) have enabled us to study both the effect of weak flares on the corona of active stars and the spatial properties of coronal flaring loops. The results are consistent with interpreting stellar flares as scaled-up versions of solar flares and show multiple evidence for flares being an important heating agent of the outer atmospheric stellar layers.