Recently, designers have been embedding reusable modules to build on-chip systems that form rich libraries of predesigned, preverified building blocks. These embedded cores make it easier to import technology to a new system and differentiate the corresponding product by leveraging intellectual property advantages. Most importantly, design reuse shortens the time-to-market for new systems. The attributes that make system chips built with embedded IP cores an attractive methodology-design reuse, heterogeneity, reconfigurability, and customizability-also make testing and debugging these chips a complex challenge. The authors review the various alternatives for testing embedded cores and describe solutions and proposed standards that are expected to play a key role in developing the core based design paradigm.