Mac OS X is Apple’s effort to create a best-of-both-breeds operating system which directly leverages the advances and experience Apple has in the user experience arena and the well-established power of BSD. Mac OS X begins with a BSD foundation called Darwin. On Darwin we build the programming toolkits from Mac OS which we call Carbon, which in turn enables us to layer on the new user interface known as Aqua. Carbon also provides a straightforward transition strategy for our existing application base. We also provide a toolkit called Cocoa, which is an object-oriented API derived from NeXT’s OpenStep. — The Challenges of Integrating the Unix and Mac OS Environments