In the waning years of the last century, there was no war on terror, gay marriage, or social media. The millennium approached, but the concept of “Millennials” was as yet unformed. The Clinton administration was implementing the new urbanist HOPE VI public-housing program; President Bill Clinton himself was being impeached.

The Charles River cleanup was progressing after Governor Bill Weld took an impromptu plunge into the river to kick-start it. And Mayor Tom Menino began feuding with City Council president Jimmy Kelly about the spoils of development on the South Boston waterfront as the economy improved.

Twenty years later, it’s clear we got some big things right. The Harbor Cleanup, Big Dig, and Silver Line have helped the city flourish in unexpected ways. But we’ve become dependent on private development to address more incremental needs, so the advantages of growth are unevenly distributed, and the fruits of development sometimes fall short of our civic aspirations. Plus we’re playing catch-up on things we missed or ignored, such as the shift to nonauto travel and especially the effects of climate change.

First, the good news. Starting 20 or more years ago, we made large public investments in water quality in Boston Harbor and the Charles River. Though motivated by environmental concerns (and a lawsuit), these investments are paying huge quality-of-life dividends today. After turning our backs for decades on our gritty postindustrial waterfront, we’re now reconceiving it as Boston’s front yard, recalling our maritime heritage.

Cleaning the harbor also helped unlock the development potential of the Seaport — more on that to follow. Some 38 miles of the 47-mile HarborWalk are in place. The mostly swimmable Charles River Basin can now be envisioned as a signal recreational resource; a swim park may not be far in our future.

The Big Dig, primarily designed to increase auto through-put, also leveraged new parks along the Charles River, in East Boston, in Charlestown, and of course downtown. The Rose F. Kennedy Greenway gets better every season with new programming and amenities. Our waterfronts will need more investment in the next 20 years to become more connected to each other and more accessible to all, but we’ve made real progress.

Another cobenefit of the Big Dig — and its companion mitigation project, the Silver Line — was better airport access, especially from the Seaport. Last year, Logan Airport had more than twice the flights it had in 1998 and more than four times as many passengers. With nonstop flights to 53 international destinations, Logan is now a portal to the world, helping explain not only General Electric’s move to the Seaport but also Boston’s increasing “global city” footprint.

But we’re also being held back by investments not yet made. Three transportation megaprojects — the Urban Ring, South Station expansion, and North-South Rail Link — have languished, though the last one could be having a second life. Even more important, we have not made public investments in many more incremental needs.

After 20 years of transit-oriented development, we have not invested nearly enough in transit, so growth is concentrated in transit-served areas and the system struggles to meet demand — not only during snowmageddons but also every weekday morning. If the Green Line extension proceeds, it will be the first rail extension since the Red Line reached Alewife in 1985, though the population of the city of Boston alone is almost 20 percent greater than it was then.

We have also avoided making the public investments that could transform the Massachusetts Turnpike in central Boston from a transportation scar to a civic asset. In the last 20 years, a succession of turnpike air rights proposals have foundered on constructability challenges, upside-down economics, and neighborhood opposition. The last air rights project, Copley Place, began almost 40 years ago and had significant public funding. It’s time to ask whether we need to deploy infrastructure support, tax abatements, or other incentives if we expect private developers to bind the wound.

The Seaport may be the most visible emblem of Boston’s resurgence. Twenty years ago, it was mostly awash in surface parking and low-scale uses. But the aforementioned troika of federally funded projects, along with land acquisition for the Boston Convention and Exhibition Center, were all completed or well under way.

To build on this momentum, in 1999 the Boston Redevelopment Authority completed the Seaport Public Realm Plan to guide the area’s growth. The first to espouse city-making principles that are now second nature, the plan envisioned mixed uses on a new interwoven street grid, with a rich public realm of sidewalks, plazas, and parks.

But the same federal government that funded the district’s emergence hobbled it with constraints. The Federal Highway Administration required the Big Dig’s surface streets to promote traffic flow rather than walkability. Federal Aviation Administration requirements after the September 11 attacks limited building heights to protect air navigation. This combination led to boxy buildings on wide streets rather than the fine-grained urbanism many hoped for.

Crucially, there was no public funding for the plan’s open space and cultural amenities; these were expected to result from development exactions. We are now seeing the fruits of this approach as the district emerges, project by project. It’s not getting rave reviews, often garnering epithets like “soulless” and “bland.”

These critiques are somewhat misguided — a new development district was never going to replicate the Back Bay. They also seem premature when neither street trees nor street retail has reached maturity. Also, architectural ambitions are increasing as development risk diminishes. (James Corner Field Operations and Rem Koolhaas are now in the mix for public realm improvements and a major building, respectively, in the Seaport Square development.)

But the real lesson is that the Seaport is the best we’re likely to get when we rely on private development to pay not only for itself but also for the local armature of streets, sidewalks, and sewers that supports it — not to mention carrying the burden of signature open spaces and cultural amenities. Development feasibility takes precedence, as it must.

Exactions have made a difference in some areas. The City of Boston’s inclusionary housing program, in place for nearly 20 years, has produced more than 4,000 affordable-housing units, fueled by a strong housing market. But relying on development to fund public amenities means that the hottest development districts get the most goodies, bypassing neighborhoods with greater needs.

The swath of central Boston from Roxbury through Dorchester to Mattapan, largely low-income neighborhoods of color, are not well served by transit or open space. They attract the least private development but have the greatest need for the benefits development can deliver. They’ll likely need more public investment to catch up with the rest of the city’s growth.

We’ve missed some other things, too. Several of today’s challenges were barely considered two decades ago, when the urban comeback story, here and elsewhere, was still tentative and “the sharing economy” and “the Internet of things” were the province of techno-fantasists.

But we certainly could have started planning sooner for post-auto mobility, by investing not only in transit but also in accommodating bikes and pedestrians. We’re also late in addressing the rising seas that could swamp the Seaport, Logan Airport, the Big Dig and Silver Line tunnels, and the new parks they brought along with them. It took Hurricane Katrina and Superstorm Sandy before we began to plan seriously for this exis­tential threat. Soon we’ll need to pivot from planning to doing.

So as gray old Boston reinvents itself again, it’s worth asking what the last 20 years revealed that might help us face the next 20. Boston, frankly, struggles with change. We love our city’s unique character and identity. They draw people here and need to be safeguarded as the city evolves. But our ardent self-regard also holds us back. In the past 20 years, we’ve relearned how cities work; we’re not clearing “slums” or building highways anymore. Yet every ambitious infrastructure proposal triggers years or even decades of often rancorous debate.

Then, when we’ve done them after all, we celebrate only briefly before slamming them for not being better. The Seaport and the Greenway may not be perfect, but most cities would die to have either one. Here in Boston, we expect to be envied, always striving to be that shining “city on a hill” of Puritan imagination. Our relentless self-criticism — the obverse of our self-regard — also holds us back.

It’s undeniable: Boston is growing and changing. It’s mostly good. If we can find ways to invest in our future and to spread the benefits of growth, we can make this a great city for everyone, for the next 20 years and well beyond.