Stars of solar type and cooler often display dramatic effects of magnetic fields in their atmosphere. These effects vary from relatively placid phenomena such as the dark photospheric spots and plages where there is enhanced emission in ionised calcium and magnesium to the much more energetic flares which erupt over short time scales. All of these aspects of magnetic activity are linked but their exact location in the atmosphere and the physical mechanism that lead to their existence remain uncertain. In this work we analyse and interpret observations of stellar and solar activity, including observations of flares and other activity-related processes. We report on spectrophotometric observations of a large flare on AT Mic and several flares on AD Leo. The latter flares were recorded with very high temporal resolution. A detailed analysis of the behaviour of the main chromospheric activity indicators is given. We also present high-resolution spectral observations of a solar flare observed during a multi-wavelength campaign. Hα images, Ca II images and magnetograms obtained at BBSO are also used in the flare analysis. We observe stellar-like behaviour in the main solar chromospheric activity indicators, which show either filling-in or emission during the flare. We have used observations and models of Balmer decrements to trace the evolution of physical parameters such as the temperature and density of the flare plasma. We also test the reliability of the procedure used. We apply this procedure to several new high temporal resolution flares on AT Mic and AD Leo as well as to flares observed during the MUSICOS 2001 campaign and by other authors, and a medium solar optical flare, observed during our solar campaign. Finally, we search for a dependence between the different physical parameters for all the flares modelled. We present optical spectroscopic observations of the binary system HR 1099 (V711 Tau) from the MUSICOS 1998 campaign. Contemporary photometric and X-ray observations performed during the campaign are also analysed and Doppler images constructed. The resulting maximum entropy reconstructions are based on the least-squares deconvolved line profiles, derived from ˜2000 photospheric absorption lines. We have obtained maps for both components of the binary system for the first time. The result is verified by using two data sets independently. (Abstract shortened by UMI.)