Eri Klas made his conductor's debut with Bernstein's West Side Story at the Estonia Opera House in 1964, and thereafter started work with the Estonian State Symphony Orchestra. As professor Haikin's assistant, Klas guest-conducted ballet performances in Moscow, and on the Moscow Bolshoy Theatre's tours in Paris, Tokyo and Athens.

In 1965, Eri Klas became a full-time conductor at the Estonia Opera House and music director in 1975-1995; the music director at the Stockholm Royal Opera in 1985-1990; from 1990 onward, Principal Guest Conductor at the Finnish National Opera. In 1991-1996, Klas was the principal conductor of the Aarhus Symphony Orchestra, in 1996 he was elected principal conductor of the Holland Symphony Orchestra and, in 1998, principal conductor of the Tampere Philharmonic Orchestra. In 1994-1997, Eri Klas held professorship in conducting at the Sibelius-Academy (Helsinki).

Eri Klas has been conducting concerts in China, Japan, Mexico, repeatedly in Australia; also in Argentina, Canada, Israel, very often in the United States, and in all major music centres in Europe; all in all, he has worked with more than 100 orchestras in more than 30 countries. Klas has been on the juries of international contests (e.g. Malko-contest in Kopenhagen); he has represented Estonia at large-scale international music and cultural events.

In 1990, Klas was awarded the Order of Nordstjernen by the Majesty of the King of Sweden. In 1992, he was presented with the Order of the Finnish Lion by the President of Finland (the First native Estonian ever, on the occasion of the 75th Independence Day of Finland).

In 1978, Eri Klas was given the honorary title of National Artist of the Estonian SSR, and in 1986, when only 46, the honorary title of the National Artist of the Soviet Union.

More than three decades of active conducting have earned Eri Klas the reputation of an internationally highly acclaimed and well-respected musician. When only 30, and still a trainee, he was asked to conduct ballet performances at the Moscow Great also on the ballet troupe's tours abroad (as far as Japan, Greece and the Philippines). Not without reason, has Klas been reputed to have been at this best when conducting ballet.

Klas was singled out among the three conductors at the Finnish National Opera opening galas as an "excellent ballet conductor" (British critic Rodney Milnes,1993). The Helsinki Sanomat wrote about the gala at the Savonlinna Opera festival: "Colours, sparkle, hot melodic lines, rhythmic explosions were abundant". As already the great Old Man of Estonian operatic singing, maestro Tiit Kuusik, baritone, repeatedly pointed out, Klas excelled when accompanying opera singers. The same has been mentioned in connection with instrumentalists - Klas' reputation when accompanying and supporting soloists is excellent. Therefore, in addition to working with famous orchestras, Klas has enjoyed the opportunity to perform with the most famous soloists of the world.

A further proof of Klas' reputation as a ballet/opera conductor is the fact that he was elected (and allowed) to work as the principal conductor of the Swedish Royal Opera (1985-1989) in very complicated circumstances - Estonia was still a part of the Soviet Union, and to be allowed to work abroad was not a rule, but an exception. Klas has always been a highly estimated and much expected guest in Finland (to conduct opera/ballet and concerts), and he has been conducting in Denmark for more than 20 years.

Klas' first appearances with a number of orchestras have been foreplanned for a long time - his reliability is a guarantee for managers, as his name means success and full audiences (e.g. with American National Symphonic Orchestra in Washington and many others); his debuts have been broadcasted live. Klas became first internationally known and respected when working in Scandinavia, then in Germany - Klas' first appearance with the famous Berliner Philarmoniker already in 1989 gained excellent reviews; he also appeared with the famous Munchen Philharmoniker (and recorded the world premiere of Schnittke's Cello concerto with Natalia Gutman in 1986), and at the Hamburg Staatsopera. Klas' geography expanded to Holland which is famous for its very good orchestras, then to the Belgian National Orchestra, RAI Torino (Schnittke-festival), the Toulouse Orchestra de Capitolium.

Performances with American orchestras over the last years form a separate chapter in Klas' biography. In 1991, he made his debut with the famous Los Angeles Philarmonics, conducting both their regular and the popular Hollywood Bowl summer concerts. "With Klas, the Philarmonic Rises Above the Routine," reads the headline of criticism in the Los Angeles Times (1993), and the critic continues: "Klas displayed a rapport with the orchestra that made for tight, sensitive and elegant performances," and furthers it by saying, that "Klas and the orchestra gave full-bodied yet careful support" to soloists. Again and again, the critics emphasize spirited, colourful, expressive performance; Klas interpretation of Shostakovitch' fifth symphony excelled for "his penchant for clean, sharply defined sound, crisp articulation, and dramatic sculpturing of the architectural detail," writes the Buffalo News (1996). "Klas' conducting is marked by a large vocabulary of gesture, a sensible approach to tempos, a dependable sense of rhythm and a streak of poetry; in the Russian and Northern European pieces Klas is clearly a master," writes The Sun (USA, March 1993) after a concert with the Baltimore Symphony orchestra. "Klas is definitely in the front rows of the musicians of the world," writes The Australian (1993). And The Sydney Morning Herald adds, "The concert with Eri Klas was something special: the performance reflected the vitality of the work, its grandiosity and expressivity."

His wonderful ability to make contacts, quick reaction, unusual readiness to communicate helps Eri Klas in his job at every step - he is constantly fighting to gain a better result. "Eri Klas manages to make the orchestra give its best, and an orchestra's best is already something extremely good" (Nya Tidningen, Uppsala, 1988). Eri Klas could also take a young, mobile group and raise it to the international standard. This is what happened at his performances with the World Youth Choir and with the Sibelius-Academy Symphony Orchestra; an unusual success was Eri Klas' work with the Asian Youth Orchestra in 1993, when they gave 13 successive concerts together in the capital cities of Europe and Asia (with Gidon Kremer as the soloist, and with the best of criticism) and finished at the famous Schleswig-Holstein festival in Germany. Klas' debut with the Berliner Philarmoniker was broadcasted live, and The Tagesspiegel wrote: "The concert was followed by rapturous applause, especially after Tchaikovsky's Piano Concerto no 1; Schnittke's Cello Concerto, conducted convincingly and with force, was an excellent achievement (soloist Natalia Gutman).

Eri Klas has been conducting many world premieres, first in Estonia, and later in his career all over the world - composers seem to sense that Klas is a reliable first performer, that his interpretation will give the work a good starting position for future international recognition. Klas had an especially close co-operation with the late, world-famed composer Alfred Schnittke. Besides many earlier works, Schnittke trusted him with the manuscript of his ballet Peer Gynt, commisioned by the well-known ballet leader of the Hamburg Staatsopera, John Neumeier. The ballet was premiered in Hamburg in 1989. "Schnittke had the luck to have the combination of a competent, sensitive and meticulous conductor on the stand in the person of Eri Klas". The Estonian musician led the Philarmoniker securely through all the shades of sound and all the levels of meaning" (Hannoverische Allgemeine, 1989). More than 30 reviews followed. Later, in 1992, Eri Klas conducted the Swedish premie're in the Stockholm Royal Opera. He has also been considered as one of the world's best conductors of Mahler's work (according to a Danish source in 1995); John Neumeier trusted him with his new ballet performance The Ninth Symphony - Mahler (Hamburg, 1994).