1.1 ,This guide covers general guidelines for the in vivo assessment of tissue-engineered medical products (TEMPs) intended to repair or regenerate bone. TEMPs included in this guide may be composed of natural or synthetic biomaterials (biocompatible and biodegradable) or composites thereof, and may contain cells or biologically active agents such as growth factors, synthetic peptides, plasmids, or cDNA. The models described in this guide are segmental critical size defects which, by definition, will not fill with viable tissue without treatment. Thus, these models represent a stringent test of a material’,s ability to induce or augment bone growth.