<p>Ink formulation is a critical factor determining the success of printed electronics (PE), yet it is often treated as a secondary step rather than a primary design challenge. This review presents ink design as an interdependent system where rheology, interfacial behavior, solvent dynamics, and curing processes must align with printing methods, substrate properties, and multilayer structures. Key formulation parameters such as viscosity, surface tension, and drying behavior are analyzed across major printing technologies, and their relationships to common defects are established. The study emphasizes that early formulation decisions, particularly binder–substrate compatibility and solvent selection, strongly control final performance and cannot be corrected at later stages. By linking material properties to process requirements and device performance, the review provides a practical framework for designing inks that are both functional and scalable. The work supports a shift from trial-and-error approaches toward more systematic and predictive ink development for next-generation printed and flexible electronic systems.</p>