A prominent glass-walled tower on the Paterson Silk Retail building was toppled earlier this month, altering the two-story expanse of glass and brick designed by modernist architect Morris Lapidus to make way for a Bank of America branch at the edge of Union Square. For some, the demolition was a victory; for others, an outrage. The disagreement is merely the latest in a debate among preservationists over the importance of architecture from that era.

Not more than 50 years ago, the goals of preservationists seemed clear: Save the old, rally against the new. The “new,” at the time, was modernism, an architecture movement characterized by boxy, glass-and-steel towers, and austere facades. At its peak, modernism became a brawny symbol of New York’s economic might during the 1950s and ’60s, producing such icons as the Lever House and the Seagram building. At its worst, it was an emblem of the mid-century building boom that resulted in the bulldozing of many historic 19th-century neighborhoods, culminating in the landmarks law of 1965. Preservationists often thought of it as the latter.

In recent years, modernism, too, has grown up. Buildings once considered architectural innovations, have become eligible for landmark status following a 30-year waiting period. Like their 19th-century predecessors, the buildings are no longer safe from the wrecking ball, causing a rethinking of preservation itself.

“We have gotten so used to modern architecture as the enemy that we have forgotten what it stood for,” an architect and director of historic preservation at Columbia University, Paul Byard, said. “And what it stood for is most of the 20th century. Whether good or bad, we have a lot to learn from it.”

Among the buildings that have recently become eligible are the CBS Building (1965) by Eero Sarrinen, who also designed the much-admired TWA Terminal; Lincoln Center for the Performing Arts (1962-1968); One Chase Manhattan Plaza (1960) by Skidmore, Owings & Merrill; the former Summit (1961) by Morris Lapidus, and the highly controversial Huntington Hartford museum (1965) at 2 Columbus Circle by Edward Durrell Stone.

While many of the buildings have drawn attention lately, from the press and preservation advocates alike, a new crop of buildings are set to turn 30 within the next decade. Unlike the ones before them, those coming of age are, at best, icons of high modernism in the 1970s and ’80s, a period typified by glass-clad skyscrapers. Many of them have since become integral pieces of New York’s skyline: the AT&T/Sony Building (1984) by Philip Johnson and John Burgee, the Citicorp Center (1977) by Hugh Stubbins and Emery Roth, and Der Scutt’s Trump Towers. Others are deteriorating or hopelessly out of date.

There are more than 1,200 buildings designated landmarks in the city, and only 10 or 12 of them squarely fit into the definition of modernism, the co-chair of the Working Architecture Group, John Jurayj, said.

In last decade, though, a number of modern architectural preservationist groups have started up in New York to lengthen that list. Among them are the Working Architecture Group and the New York chapter of Docomomo, or Documentation and Conservation of the Monuments of the Modernist Movement.

“People associate preservationists as being reactionary and not proactive and lying down in front of bulldozers at the moment,” the co-chairwoman of Docomomo’s New York-chapter, Nina Rappaport, said. “The truth is that we are actually out there fighting for these buildings years in advance and trying to identify them before they become eligible.”

These groups largely work as advocates, helping to educate the public about the significance of modern structure.

“Much of preservation is associated with brownstones and these iconic images of what historic communities look like, and modern buildings don’t always conform to this,” the executive director of Landmarks West!, Kate Wood, said. “But we think they are significant and remind us where we have been as a culture.”

Their efforts have resulted in the creation of several preservation watch lists, one authored by architect Robert A.M. Stern in 1994. Another was offered to the public last spring, after a section of the much-admired Bronx Development Center by Richard Meier was demolished, a couple of years shy of its 30th birthday. “When that went down, we knew we had to start keeping track of these buildings,” a senior vice president of the Municipal Arts Society, Frank Sanchis, said. “At this age, they fall out of fashion and become vulnerable to demolition. It is too bad it had not become eligible sooner. It would have certainly been a landmark.”

The incident prompted the Municipal Arts Society, New York’s leading preservationist organization, to launch a citywide campaign, 30 Under 30, to identify buildings both worthy and eligible of landmark status.

“When they are first built, it is difficult to put them into perspective or see how they fit into the arc of architectural history,” Ms. Wood said. “Thirty years later, you are skipping a generation and beginning to look at these buildings with fresh eyes.”

But even after 30 years, without the benefit of hindsight, selecting a list of historically significant buildings is a thorny, often volatile task involving judgments on style, aesthetics, and significance. “It takes a lot of strength and wisdom to look at buildings in a style you personally don’t like,” a former fellow at the Municipal Arts Society and curator of 30 under 30, Vicki Weiner, said. “It is much harder than looking at 100-year-old buildings that have stood the test of time.”

The director of the city’s Landmarks Preservation Commission, Robert Tierney, faces the task of helping to decide which buildings are to be given landmark status.

“After 30 or 40 years, you don’t always have the immediate ability to judge the long-term significance,” Mr. Tierney said. “I mean, when we designate these things, that is it. It is permanent.”

Such is the case with Edward Durrell Stone’s Huntington Hartford Museum at 2 Columbus Circle, an austere white marble slab, once dubbed by the architectural critic Ada Louise Huxtable as the “lollipop” building.

The Museum of Arts and Design intends to buy the building from the city and give it a complete overhaul. But a small group of activists are now suing the city to make it a landmark. They argue that its historic significance trumps its aesthetic value, an argument that baffles those who have long considered it an eyesore.

Another building on the watch list has also stirred considerable debate in the last few years: the Marriott Marquis in Times Square, a modernist structure that rose on the rubble of the beloved Helen Hayes Theater. “Sure, we don’t like the building, but the Marriott Marquis represents a pivotal historical decision in developing the West Side,” Mr. Byard said. “You have to make the distinction between buildings for which there is a great deal to learn from and those that are just mediocre. To single out Columbus Circle is a wonderful display in ignorance.”

Advocates for modern architectural preservation argue that the issue is not as much about likeability, as it is about preserving a certain historical era or architectural moment against the swinging pendulum of fads and trends.

“We are always reacting to the pervious generation, and then 20 years later, a generation goes by and you look back, and you were sorry that you lost something,” Ms. Wood said, citing Penn Station as an example. “When Penn Station was demolished in 1963, it was about the same age as 2 Columbus Circle. It was sort of outdated, a white elephant of a building that took up an entire city block. And it was cleared away for modern tower. Now I think we regret that decision.”

Similar arguments have also been made about the World Trade Center towers. “It was a building that everybody loved to hate. But I am sure people wish it was still there,” Mr. Jurayj said. “You can argue back and forth about whether or not it was a great building, and that is just aesthetics. But it is a strong example of what happened to us in the 1970s, for better or worse.”

The preservationist movement has yet to claim any overwhelming victories for modern architecture, aside from obvious modernist icons like Mies de Van der Rohe’s Seagram Building and the TWA Terminal. The task of reviewing candidates falls to the landmarks commission, which has a backlog of proposals dating back to the 1960s. Each year, the landmarks commission sifts through more than 400 proposals in order to designate 16-20 historic buildings or districts, a process that is largely undertaken by volunteer members, Mr. Tierney said.

“As time passes, more modern buildings will come into our inventory, and we will see more designations,” Mr. Tiernery said. The commission will also see more candidates, especially considering the recent surge of construction in and around ground zero and along New York City’s waterfronts. About this, Mr. Tierney said, “There is a kind of architectural renaissance taking place right now with some of the great architects, like Norman Foster and Renzo Piano, starting to build here now.

“And it is very exciting. I would like to be landmarks commissioner 30 years from now.”