"Twenty years now; where'd they go," Bob Seger wistfully growls in "Like a Rock," his epic paean to departed youth. "Twenty years, I don't know. I sit and I wonder sometimes where they've gone."

And well should he wonder. It has been 18 years since Seger broke through to stardom with the single and album "Night Moves," and 25 since he first emerged from Detroit, earning national notice with "Ramblin' Gamblin' Man." This is a career most deserving of a proper retrospective . . . and "Greatest Hits" almost fits the bill. But not quite.With his gritty alto sax of a voice and a gift for solid ballads that remarkably mesh memory, passion and rock 'n' roll, Seger became America's blue-collar troubadour in the late '70s and intermittently through the '80s. "Hollywood Nights." "Still the Same." "Old Time Rock & Roll." "Against the Wind." It all jelled with the classic "Night Moves," that great voice telling a tale, girded by acoustic guitar, rippling piano and a female backing chorus.

This collection is paced with a dozen great performances and two new ones (Chuck Berry's "C'est La Vie" and "In Your Time," a loving song of anticipation looking forward to the live of his young son). And some of Seger's short notes about the songs are wonderful. For example, after they'd recorded "Hollywood Nights," Little Feat keyboardist Bill Payne asked for a tape of the song to play on the way home. "He called me the next day and said while he'd been listening, he looked down and found himself going 100 miles an hour on the freeway."

Great stuff. Great Seger.

But where is that breakthrough "Ramblin' Gamblin' Man"? Where are "Get Out of Denver" and "Fire Lake"? And how can this be Seger's "Greatest Hits" without his two biggest, his sole No. 1, "Shakedown," from "Beverly Hills Cop II," and his unforgettable cover of Rodney Crowell's "Shame on the Moon," a No. 2?

Sometimes only a boxed set, or at least a double-disc set, will do. This is one of those times.

BOB SEGER & THE SILVER BULLET BAND, "Greatest Hits," including: Roll Me Away, Night Moves, Turn the Page, You'll Accomp'ny Me, Hollywood Nights, Still the Same, Old Time Rock & Roll, We've Got Tonight, Against the Wind, Mainstreet, the Fire Inside, Like a Rock, C'est La Vie, In Your Time.