Plasma-based seeded soft x-ray lasers (PBSXRL) have the potential to generate high-energy, fully coherent, short pulse beam. Nowadays, PBSXRL have demonstrated experimentally 1 mu J, 1 ps (1 MW) fully coherent, aberration-free pulses. Nevertheless, most exciting applications (as single-shot coherent imaging) require pulses shorter than 200 fs and energy ranging from 10 mu J to several miliJoules. In this chapter we will review the theoretical modeling tools and the results obtained related to this source.