This book explores the possibility of combining spin and quantum fluids, often associated with profound and intriguing phenomena in modern physics, into a single conceptual framework - the "spin fluid". Within this context, the book examines physical systems falling into two broad categories:Spinor Condensates: These are systems inspired by atomic Bose-Einstein condensates that possess internal spin degrees of freedom. Related systems include the superfluid phase of helium-3 (He-3A) and, from electronic systems, the double-layered quantum Hall fluid in two dimensions. Polarized Light in Nonlinear Optics: Here, we treat polarized light as a fluidic object and the polarization state of light as a spinor-like degree of freedom. In the framework of quantum optics, the light field can be interpreted as an order parameter for a photon condensate, despite photons being massless. A recurring theme of the book is the role of topological defects, which provide a unifying structural feature across these diverse systems.