This Liber Amicorum---"Book of Friends"---, a tribute by many of his friends, colleagues, collaborators and former students, celebrates the 85th birthday of Professor Arto Salomaa. 

Arto  has for half a century inspired and guided the foundations of computer science and has been a central figure in modern formal language and automata theory. He is among the most decorated computer scientists, the results of his work are embedded in the fabric of theoretical computer science,  and his influence on  researchers in theoretical computer science all over the world  has been fundamental. 

In his research, Arto is known for being prolific, deep, and broad. Since 1959, he has had more than 500 publications, including 50 books. While this is unusual by any standard, it is exceptional in theoretical computer science. In terms of areas, he has made major contributions to, among others,  the theory of formal languages and automata, grammars and rewriting systems, combinatorics on words, cryptography, authentication and secret sharing, models of computation, algorithmic information theory, many-valued logic, Boolean functions,  DNA computing, and reaction systems.

Among  Arto's well-known books, a few are especially notable: "Formal Languages," published in 1973, is a brilliant exposition of the mathematical theory of formal languages that has inspired generations of computer scientists.  Indeed, anecdotal evidence abounds that  "Formal Languages" has been called by many the most reliable source of correct and clear proofs in formal language theory. In 1991, this book was credited as being one of 100 most cited texts in mathematics and declared a classic by the Association for Computing Machinery. More recently,  the "Handbook of Formal Languages" in three volumes,  with Grzegorz Rozenberg, has become the standard reference for the field. Other books have presented galvanizing studies of new areas, as they have emerged to prominence.

Finnish sauna has a special role in the world of Arto.   Where some characterize problems in computer science in terms of algorithmic complexity, Arto characterizes problems in terms of  sauna sessions: "[...] I have an idea that the veins in your brain open when you are in sauna. And then, if a problem is difficult, like Sherlock Holmes spoke of three-pipe-problems, we spoke of three-sauna-problems [...],  and that you have to go three times to sauna to solve this problem."

Arto has perhaps had the greatest impact through  his influence on other scientists.  His way of thinking and intuition of which areas will become important in the future and which will be fruitful, have always been inspiring and  most prized.   Arto has scientifically educated, coached, and imparted life lessons -- in  his characteristic style of terse nuggets of wisdom,  uttered  in a friendly but decisive baritone voice -- to  countless students, colleagues and collaborators. By now, Arto is a real äijä (Äijä is Finnish for "grandfather.") to many. 

The combination of his impressive research record with  an "efficient, friendly professionalism",  a larger-than-life presence, fortitude,  and integrity, all permeated by  his ever-present kindness towards one and all,  uniquely  qualify Arto  for the title of Friendly Towering Figure of Theoretical Computer Science.