Various approaches to the production of soft X-ray in the water window (2.3～4.4 nm) and the nearby wavelength range are discussed. Atomic number of the material used as target is the most fundmental factor which affects X-ray production and conversion efficiency. High intense X-rays can be realized with laser plasmas in nonequilibrium or in equilibrium state, and that X-rays from black body sources produced by laser plasmas in equilibrium state be used for X-ray microscopy of living samples is proposed. With the present lasers, X-ray microscopy can be made using narrow band emission or black body radiation from laser plasmas. The black body X-ray source produced by high intense femtosecond laser interaction with solids of high atomic number is useful in time-resolved spectroscopy as well as in X-ray microscopy.