In the early 1960s, Kleinrock pioneered the application of queueing theory to model delays in message switching networks in his Ph.D. thesis, published as a book in 1964.

His theoretical work on hierarchical routing in the late 1970s with student Farouk Kamoun remains critical to the operation of the Internet today.

Leonard Kleinrock was born in New York City on June 13, 1934, to a Jewish family,[3] and graduated from the noted Bronx High School of Science in 1951.

He then joined the faculty at the University of California at Los Angeles (UCLA), where he remains to the present day; during 1991–1995 he served as the chairman of the Computer Science Department there.

[5][6] Donald Davies, in his 1966 paper on packet switching, applied Kleinorck's techniques to show that "there is an ample margin between the estimated performance of the [packet-switched] system and the stated requirement" in terms of a satisfactory response time for a human user.

[9] Roberts asked Frank Westervelt to explore the questions of message size and contents for the network, and to write a position paper on the intercomputer communication protocol including “conventions for character and block transmission, error checking and retransmission, and computer and user identification.

"[10] Later that year, Roberts learned about packet switching from a paper written by Davies, presented at the October 1967 Symposium on Operating Systems Principles, and incorporated the concept into the proposal for the ARPANET.

[5][16][17][18] In addition, Kleinrock managed the software team at UCLA — including Steve Crocker, Jon Postel, and Vint Cerf — who developed the host-host protocol for the ARPANET, the Network Control Program (NCP).

[28] Kleinrock's theoretical work on hierarchical routing with student Farouk Kamoun remains critical to the operation of the Internet today.

[52] Historians and the U.S. National Inventors Hall of Fame recognize Baran and Davies for independently inventing the concept of digital packet switching used in modern computer networking including the Internet.

In 1980, he was elected a member of the National Academy of Engineering for pioneering contributions to the field and leadership as an educator in computer communications networks.

"The 2007 National Medal of Science to Leonard Kleinrock for his fundamental contributions to the mathematical theory of modern data networks, and for the functional specification of packet switching, which is the foundation of Internet technology.

The designation of Eminent Member is the organization's highest membership grade and is conferred upon those select few whose outstanding technical attainments and contributions through leadership in the fields of electrical and computer engineering have significantly benefited society.

Leonard Kleinrock was given the 2014 BBVA Foundation Frontiers of Knowledge Award "for his seminal contributions to the theory and practical development of the Internet," in the words of the jury's citation.