Described herein are injection devices capable of automatically injecting substances into the soft tissue of a patient. The devices can inject low to high viscosity materials at predetermined, user selected injection rates, allowing the operator more control than a traditional syringe. The devices can allow mixing of more than one substance and/or reconstitution of a solid substance for injection. The injection devices described herein can allow the operator to easily inject one or more low to high viscosity liquid or gel soft-tissue augmentation fillers, one or more drugs, one or more other biocompatible materials, or combinations thereof.