Many investigations performed in mice (Mus musculus) require the administration of an experimental agent to the animal. For example, it may be of interest to test the efficacy of a specific therapy, to induce a pathologic condition, or to administer anesthesia or palliative care. In order to ensure safe and efficient delivery, it is important to consider a variety of factors prior to the administration of the treatment. This video, which reviews agent administration in the mouse, begins by highlighting properties to consider, such as viscosity, dose, and palatability, when planning the administration of an experimental agent. The subsequent discussion focuses on injection methods, including delineation of the structural components of the syringe and needle, how to interpret needle gauge, and safe mouse restraint methods for common injection sites. Detailed instructions are provided for performing subcutaneous (SC/SubQ), intraperitoneal (IP), and tail vein (IV) injections in mice. Furthermore, applications of these techniques as well as alternative administration routes are discussed.