Theories of a technique for compression of an ultrashort femtosecond laser pulse beyond its Fourier-transform limit based on the pulse shaping technique are proposed. The technique is called superresolution in time domain (STD). Global optimization theories for the design of a mask to modulate the spectrum of an input pulse to obtain a STD output pulse are proposed. Several design examples illustrate the feasibility of STD. Some fundamental limits of STD are also provided. (c) 2006 Optical Society of America.