This chapter is to describe and compare the original Unix developed at AT&T Bell Laboratories and Solaris 2 long descendant of Unix. The features of original Unix are taken from [1], [2], and [3]. Strictly speaking these article are not about Unix on PDP-7, on which the first development was done, but about one on PDP-11. However these articles are most representative documents on the early version of Unix, provide us thorough understanding of the Unix, hence the author depends on their contents. On the other hand, the author had difficulty to gather enough information about Solaris 2 . So far only [4], [5], [6] are available. In addition most of the features of Solaris 2 also exist in Unix SVR4. Hence although the title of this chapter is Comparison between Original Unix and Solaris 2 , some of its contents will be concerned with the the difference between the classical Unix and the modern Unix. The section of networking (1.6) is less theoretical and technical than other sections. This is because we have not finished the networking and distributed system part in the class and hence the author's degree of understanding on the networking is lower than other aspects of operating systems.