Americans have long had a passion for climbing family trees, from watching ““Roots’’ to mailing away for supposed proof of ties to the crowned heads of Europe. But in the past few years, genealogy has gone from spectator sport to major-league passion. A 1995 survey for American Demographics magazine found that 42 million Americans have started to trace their heritage. Everton’s Genealogical Helper magazine receives more than 1,500 books to review every year. Last month Broderbund sold the one millionth copy of its Family Tree Maker computer program. The Internet has also made it easier to track down everything from birth and death certificates to people with similar surnames. Baby boomers, on the verge of becoming grandparents, have become heritage hungry. So have Americans of East European descent, who have new access to documents behind the old Iron Curtain. Says Fran Shane of the National Genealogical Society: ““Whenever I go to a party, I always have people cornering me and saying, “How do I get started?’ ''

It used to be that only Mayflower descendants had the blue-blood obsession with ““lineage’’ to ask that question. But newer immigrants are now glancing over their shoulders, too. ““Within three generations, they get unashamed of their own past and comfortable enough with their Americanness to look back,’’ says Lynn Betlock of the New England Historic Genealogical Society. Baby boomers, who seem to turn their every passion into a national pastime, have had an especially strong hand in the genealogy boom. That may stem from a desire to get their houses in order for the millennium or, more likely, the mortal longings that come from having kids–and watching them leave home.

But the biggest boon to the heritage hunt has been cyberspace. No one has been more influential there than Cyndi Howells, a Puyallup, Wash., housewife who became obsessed with genealogy after tracing her own family tree. Her home page includes 14,650 links to information esoteric and banal (chart). And it’s free, as are online chat rooms and bulletin boards that can point sleuths to countless ethnic-oriented Web sites. Many amateur genealogists start with commercial CD-ROMs, such as the $69.99 Family Tree Maker. It organizes information and provides 115 million names culled from census data, marriage indices and birth records.

But experts warn that the computer isn’t a one-stop genealogical shop. Databases and Web sites can direct a search, but libraries and courthouses still contain the essential primary-source information, such as birth certificates, real-estate deeds and wills. And just because information exists on the Net doesn’t mean it’s right. ““It would be terrible to spend five years tracing the wrong people,’’ says Kay Germain Ingalls, president of the Association of Professional Genealogists. (Like many genealogists, Ingalls uses her maiden name–in case a long-lost relative spots it.) Professionals charge from $7 to $100 an hour, with a typical search costing up to $1,400–though people have spent up to $300,000 for centuries-old histories.

Some ethnic groups face more challenging searches than others. Irish-Americans are frequently stuck for pre-1900 data. In 1921, a major public-records office in Dublin burned down, and many census documents were reduced to pulp during paper shortages. European Jews also have difficulty finding their ancestors, since the Nazis burned records kept in synagogues. But the rulers of 19th-century Eastern Europe kept very careful tabs on their subjects, and many of those documents have recently become available with the dissolution of the Soviet bloc.

The most difficult ancestry to trace may be African-American. Because owners often forced their surnames onto their slaves–and sometimes reproduced with them–blacks have to unravel the white family’s heritage to discover their own. Lynching records, evidence of human branding and family Bibles also play a role in black genealogy. But not all African-Americans were slaves. Katherine Butler Jones began researching her roots when she happened across her great-grandparents’ marriage certificate folded in an envelope in her Harlem apartment. Jones discovered that her great-great-grandfather owned a farm in upstate New York in 1827, and every generation thereafter also owned land. Discovering that she’s part of the ““fabric of this country,’’ says Jones, 61, ““is a very good feeling.''

Perhaps the most avid–and successful–ancestry hunters are the Mormons. Because Mormons even hold baptisms for ancestors who died before the Church of Jesus Christ of Latter-Day Saints was founded in 1830, they have amassed records (open to everyone) on more than 2 billion people of all faiths. It was the Mormon archives that helped Darlene Baker piece together her past. Baker couldn’t find much information on her maternal grandfather, Melvin Overly, who she believes wouldn’t talk about his ancestry because his mother and three siblings died when he was a boy. The church files and some computer sleuthing led to an 87-year-old relative in Seattle, who produced a suitcase of family photos and countless stories, including the fact that Melvin’s father was a drunk. ““She told me that the horse and buggy would come home without him since he had fallen off,’’ says Baker, a 51-year-old hairdresser from Utah. ““I thought, I can feel his pain. I might have become a drunk, too. He lost most of his family in just one year.’’ The bittersweet irony is that in one visit with a long-lost relative, Baker got her family back.

Genealogy is simply a matter of collecting pieces of a puzzle that can illustrate your heritage. It’s now a vastly popular, high-tech hobby. A guide to getting started:

Acquire pedigree charts or “family group” sheets, available at genealogy Web sites and research centers. These are similar to the flow charts that professionals use to keep information across generations.

Start with yourself and work backward, plugging into the pedigree charts everything you know about yourself, your parents and your grandparents.

Interview extended family. Flesh out stories; ask when and where ancestors were born, married and died.

Search family attics. Look for family Bibles; birth, death and marriage certificates; old letters; diaries; inscribed dishes; immigration papers, and coats-of-arms.

The 1,540 Mormon-operated Family History Centers are arguably the best one-stop shopping for amateur genealogists. They have computer links to the world’s largest genealogical database, The Family History Library in Salt Lake City (800-346-6044).

State libraries collect county records and old newspapers.

County historical societies house old county maps and histories.

Courthouses provide marriage and probate records.

Public libraries keep county and state censuses.

The country’s dozen National Archives keep census, naturalization, federal court and shipping and navigation records.

Cemeteries contain hard-to-find death dates, and active cemeteries point to who bought the plot.

“Ancestors” ($16; Houghton Mifflin)

“The Source” ($63.93; Ancestry)

Family Origins ($29.95; Parson’s Technology)

Family Tree Maker ($69.99; Broderbund Software)

Cyndi’s List of Genealogy Sites (www.oz.net/~cyndihow/sites.htm)), 14,000 links, specializing in ethnic searches

Ancestry Search (ancestry.com)

soc.genealogy.mis, bulletin board with genealogical discussion

soc.genealogy.surnames, for surname queries

Adoptees’ Liberty Movement Assn. (212-765-2861)

Hidden Child Foundation (212-885-7900) finds relatives lost in the Holocaust

Professional genealogist research fees range from $7 to $100 per hour