Due to network effects and switching costs, platform providers often become entrenched. To enter established markets, aspiring providers of new platforms generally must offer revolutionary functionality. We explore a second path to entry that does not rely on Schumpeterian innovation: platform envelopment. By leveraging shared user relationships and common components, one platform provider can move into another's market, combining its own functionality with the target's in a multi-platform bundle. Dominant firms otherwise sheltered from entry by standalone rivals can be vulnerable to an adjacent platform provider's envelopment attack. We develop a taxonomy of envelopment attacks and analyze conditions under which they are likely to succeed.