Understanding the interactions and organization of various molecular constituents of cartilage is an essential prerequisite to designing and developing effective nuclear magnetic resonance and magnetic resonance imaging (MRI) strategies to characterize the state of this tissue in normal and abnormal development, aging, health and disease, and trauma. In this chapter the osmotic properties of cartilage are discussed. An attempt is made to relate the tissue’s macroscopic behavior to its hierarchical organization and the physical–chemical interactions among its main macromolecular constituents. It is shown how to separate the osmotic contribution of the proteoglycan assemblies from that of the collagen network. The interactions between the main macromolecular components of cartilage extracellular matrix (ECM) are determined using an array of complementary experimental techniques (osmotic pressure measurements, small-angle X-ray scattering, small-angle neutron scattering, dynamic light scattering, and atomic force microscopy) probing the ECM structure and dynamics over a broad range of length and timescales. This knowledge is also important to construct magnetic resonance phantoms for quantitative MRI that exhibit osmotic, mechanical, and relaxation properties similar to that of cartilage. Such biomimetic phantoms with well-characterized physical and imaging properties are critically important to validate MRI characteristics.