Oral liquids—principally solutions and suspensions—are among the most widely used pediatric formulations. They enable flexible dosing and are swallowable by children of all ages, and can also be administered through feeding tubes if necessary. The principal challenges in oral liquid formulation selection include the solubility of the drug, the physical and chemical stability of the product, the need for microbial presentation of multidose presentations, the taste of the formulation, and the usability of the product by the parent or caregiver. This chapter reviews the principles of formulating oral solutions and suspensions, as well as their packaging. Also discussed are considerations of biopharmaceutics and dosing devices, as well as the particular formulation challenges associated with poorly soluble drugs and modified-release products. This chapter highlights the complexity of pediatric formulation selection, the trade-offs inherent in this decision, and the complexity of the formulation-dosing device interface.