Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for dynamic sandboxing. In one aspect, a method includes obtaining software including program instructions for execution by a data processing apparatus; identifying, from a plurality of predefined software characteristics, software characteristics of the software, each identified software characteristic corresponding to a set of the program instructions included in the software; accessing sandbox data defining, for each of the predefined software characteristics, one or more sandbox methods that are eligible for use when executing program instructions that correspond to the predefined software characteristics; selecting, from a plurality of sandbox methods, one or more sandbox methods based on the identified software characteristics and their respectively corresponding sandbox methods, each of the plurality of sandbox methods comprising sandbox instructions for executing program instructions using fault isolation techniques; and causing execution of the software using the selected sandbox methods.