 Thanks are due to Peter Norvig and Prentice-Hall for allowing figures from AIMA to be used in this entry. Thanks are due as well to the many first-rate (human) minds who have read earlier drafts of this entry, and provided helpful feedback. Without the support of our AI research and development from both ONR and AFOSR, our knowledge of AI and ML would confessedly be acutely narrow, and we are grateful for the support. We are also very grateful to the anonymous referees who provided us with meticulous reviews in our reviewing round in late 2015 to early 2016. Special acknowledgements are due to the SEP editors and, in particular, Uri Nodelman for patiently working with us throughout and for providing technical and insightful editorial help. 