Of all the face-offs and provocative pairings that enliven the Art Institute of Chicagos new Modern Wing, perhaps the most striking is the vista, from every floor of Renzo Pianos elegant 264,000-square-foot addition, of the Frank Gehry band shell in Millennium Park. It’s as if Piano, the current favorite among museum builders, were paying homage to the rising starchitect of the last century. Or perhaps it’s the other way around: In its relatively modest size, the Gehry structure seems to acknowledge Piano’s ascendance as light and grace triumph over Baroque excess. Either way, it’s a neat showdown, even when the windows in the new space are covered with view-muting, floor-to-ceiling white screens during the day.

The addition, which took 10 years to build, at a cost of $294 million, also looks out on a generous slice of the city’s downtown skyline, a reminder that Piano owes much to the steel-framed skyscraper and to two earlier masters whose talents flourished in Chicago: Mies van der Rohe and Frank Lloyd Wright. (There are even plantings of prairie grass in the ground-floor Margot and Thomas Pritzker Garden, perhaps a reminder of the latter’s early style, and the staircase, suspended on slender steel rods, was reportedly inspired by the former’s design for the Arts Club of Chicago.) Sited behind the Institute’s stately Beaux Arts building on Michigan Avenue, the new wing rises three stories and increases the museum’s size to more than a million square feet, making it the second largest art museum in the country, behind New York’s Metropolitan Museum of Art. A 620-foot-long bridge connects Piano’s restrained addition with Millennium Park, across the street, and overhanging the entrance is the most distinctive of the structure’s several green initiatives, a sunshade, dubbed a “flying carpet canopy,” that filters daylight into the upper-level gallery spaces and saves on electricity.

Piano’s previous designs, which include the Pompidou Center in Paris (in collaboration with Richard Rogers) and expanding the Morgan Library in New York, have been praised for relating well to their surroundings. But here, the new building does not seem intended to complement or continue the architectural vocabulary of the original 19th-century museum. Indeed, new and old are sufficiently separated that from the street it is almost impossible to consider them as members of the same family. More than anything, the new structure seems marked by its ability to gracefully combine weightiness and weightlessness. A series of heavy walls made of the same Indiana limestone as the rest of the museum frame the main entry and are lightened by the towering glass-and-steel façade punctuated by slender columns, while from Monroe Street the building has a temple-like assurance, even as the attenuated columns and glass façade project a feeling of weightlessness. Inside the grand central Griffin Court, where the ceiling is three stories high, comparisons can be made to an Italian galleria or a soaring cathedral.

But the real test — as for all art museums — is how well the architecture gets out of the way so that visitors can look at the art. And this the Modern Wing, for the most part, does superbly. The long central hall has already been criticized as an invitation to chaos (admittedly, the Cy Twombly sculpture installed there looks almost lost), but on Saturday, May 16, at the official opening, a generous throng seemed to be navigating that space with no difficulty.

Small complaints aside, the Modern Wing is a wondrous reminder of the importance of art to a great city’s civic life, especially in hard times. If the connection between the museum’s art of the last 100-some years and everything that came before it is physically and psychically a bit of a leap (a spartan new corridor connects the old building to the new), it’s also a reminder that the journey through art history was not without its own brave strides into the new and the uncharted.

Galleries on the main floor are devoted to a temporary exhibition of recent works by Twombly, as well as film, video, and photography, but the survey begins two flights up on the third. Installed roughly in chronological order, the works tell the familiar story of modernism and postmodernism, from Cubism to Koons and beyond, but a number of curatorial surprises give the conventional riff new life. At the entrance, three representations of the eternal feminine from the last century make up a lusty welcoming committee: Henry Moores big bronze maquette for a reclining figure (1957) and two paintings: Picassos Portrait of Sylvette David (1954) and Matisses huge Bathers by a River (1917). The generously proportioned rooms contain pleasures at every turn, like Brancusi’s little-known sculptures Two Penguins (1911–14) and Wisdom (1908) and Russian Futurist Natalia Goncharovas Spanish Dancer (1916), an elegant conflation of folk art tradition and religious icons. Even old chestnuts have never looked better: Dalis Venus de Milo with Drawers (1936) is still sexy and startling, and Magrittes Time Transfixed (1938), a locomotive busting out of a fireplace, shows an overexposed Surrealist in top form.

One of the greatest delights on this floor is the intact Lindy and Edwin Bergman Collection, a quiet haven of interior galleries that includes a wall of 38 Joseph Cornell boxes, a beguiling little predator bird from the late French sculptor Germaine Richier, and a gouache on corrugated cardboard that leaves you guessing the artist until you look at the label (it’s Picasso).

The second floor picks up with art from 1950 to the present, and here the curatorial thinking is abuzz with clever juxtapositions. A room of paintings from contemporary artists Lucian Freud, Lisa Yuskavage, Margherita Manzanelli, and Luc Tuymans gives a nod to the ongoing tradition of figurative painting. Kerry James Marshalls homage to Fragonards The Swing is laugh-out-loud funny, especially when considered in tandem with Robert Gobers deadpan objects and wallpaper. Eva Hesse and Richard Serra share a gallery, showing the masculine and feminine sides of Minimalism, and pairing Roy Lichtenstein with Andy Warhol reconsiders Pop art’s chillier moments. The museum has also given special consideration to the city’s homegrown talent, with a gallery devoted to Jim Nutt and a creepy Ed Paschke painting of a boxer glowering at Jeff Koonss porcelain Woman in Tub.

Several great landmarks from the 1950s, such as de Koonings Excavation and Joan Mitchells knockout City Landscape, are reminders of Abstract Expressionism’s ongoing power, but Color Field painting is almost entirely slighted, the exception being a pale Sam Francis canvas that seems like a hasty afterthought. Occasionally works are sited in alcoves — like Francis Bacons Figure with Meat (1954) or Martin Puryears Sanctuary (1982) — as if they don’t quite fit with the story line, but the placement also asks viewers to consider the rich individual detours from the movements that dominated the last century.

Regrettably, other galleries don’t quite yet hold their own against the strengths of the painting and sculpture collection. Twombly, in the Abbott Galleries for Special Exhibitions, is still going strong at 81, but he has produced better work than this. The Ellsworth Kelly sculpture in the Pritzker Garden looks lonely and purely decorative, and the spaces devoted to architecture and design are letdowns after the high level of curatorial panache elsewhere. But a new addition of this kind has to be an organic and malleable work in progress. Let’s hope that future tweaking will bring stronger contributions and fresher thinking to the smaller attractions as well as the main event.