The phase behavior of ABA triblock copolymers in aqueous solutions is studied theoretically, using both continuum and lattice mean-field theories. A variety of lyotropic liquid crystalline phases is found for the Pluronic family, and the effects of temperature, copolymer concentration, Overall molecular weight, and block sizes are investigated. The theoretical scheme incorporates "internal" monomeric states as well as the effects of the spatial arrangement of the molecules and may be viewed as a generalization of the traditional theory of ordered phases in block copolymers. The interaction parameters determined for poly(oxyethylene) and poly(oxypropylene) homopolymers in aqueous solutions are found to give good agreement for the theoretically predicted ordered structures when compared with measurements on different Pluronic polymers using H-2 NMR of deuterated water and small-angle X-ray scattering.