Superprotonic solid acid electrolytes, materials with chemical and physical properties intermediate between conventional acids (e.g., H₃PO₄) and conventional salts (e.g., Cs₃PO₄) have emerged as attractive candidates for fuel cell and other electrochemical applications. Key characteristics of these materials are tetrahedral oxyanion groups linked by hydrogen bonds, and a polymorphic structural transition to a disordered state at moderate temperatures. Rapid oxyanion reorientation and dynamic disorder of the hydrogen bond network facilitate high proton conductivity in the high temperature phase. Materials exhibiting a superprotonic transition include CsHSO₄, Cs₃H(SeO₄)₂, CsH₂PO₄, and Cs₂(HSO₄)(H₂PO₄). Here we review the present state of understanding of proton transport mechanisms and the factors governing the transition behavior as gathered from macroscale measurements of conductivity and thermal properties and from atomistic level studies using nuclear magnetic resonance spectroscopy. We focus in particular on CsH₂PO₄, the technologically most significant superprotonic solid acid, and its chemical modifications produced by substitution on either the alkali metal or proton site (deuterated materials). Dramatic changes in phase behavior and proton conductivity can be induced by only minor changes in chemistry, suggesting routes for tuning behavior to achieve desired outcomes.