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This thesis describes work undertaken using data from the ROSAT and ASCA missions, in order to study the coronal x-ray emission of cool, late type stars. Chapters 1 and 2 summarise the emission processes involved and the methods by which the x-rays are detected and analysed, including a discussion of the ROSAT and ASCA missions. Chapter 3 details a deep soft x-ray survey of the Galactic Plane using the ROSAT Position Sensitive Proportional Counter. The data consist of 9 fields, each of 10ks exposure, pointed at positions on or very close to the plane. Presented are the source list, number-flux relation and the x-ray to optical flux ratios. I present the results of source count modelling which suggest 50% stellar and 8% extra-galactic contributions. The nature of the unidentified population is discussed. Chapter 1 discusses the temporal variability of the x-ray emission of stars in the Hyades cluster. The data consist of 13 PSPC fields, each of 20ks exposure, targeted in the central regions of the cluster. Little evidence is present to suggest long term variability, though 14 do exhibit short term variability. Time resolved spectra of detected flares are presented. Flare modelling results suggest flare loop heights of 1-2 solar radii. Chapters 5 details observations, obtained using the ASCA satellite, of 3 sources, VW Cep. SW Lac and 44i Boo. These observations form part of an ongoing survey of W UMa systems. Spectral and temporal results are compared to a previous ROSAT survey. I show that the observed systems have a high temperature component comparable with that seen in RS CVN and BY Dra systems. I also provide evidence for non-solar metal abundances in these systems.