In this chapter, we discuss general backgrounds of our research theme: the self-organization of polymer solutions and solutions of polymer mixtures under external flow fields, specifically under shear flow fields. The systems belong to the so-called open nonequilibrium systemsOpen nonequilibrium systems that are open to various imposed external energy flows. Structures formed in such systems are called “dissipative structuresDissipative structures”, since they are self-organized in the dynamic balance between burst (and/or deformation) driven by dissipation of the energy stored in the systems by the imposed mechanical force and growth (and/or restoration) of the burst structures driven by the thermodynamic force inherent to the systems. We focus on to investigate mesoscopic-scale dissipative structuresDissipative structures spanning from a few nm to several tens μm as a function of flow rate. We investigate also the flow-induced phase transitions from two-phase to single-phase and vice versa. The investigation utilizes the rheo-optical method which enables to observe real-time and in situ small-angle light scattering and small-angle neutron scattering in reciprocal space as well as birefringence, transmitted light intensity, shear and normal stress in real space. The chapter discusses the basic physics underlying the flow-induced phase transition. Polymer systems provide a good model system for fundamental studies of self-organization in open nonequilibrium systems because of such a unique point that large structures grow or relax very slowly in the polymer systems, which brings about the following characteristics.