MIAMI—Today Terry Riley shocked the art world by announcing his decision, effective immediately, to leave the Miami Art Museum (MAM), where he’d been director for four years. Prior to his appointment at MAM, he had served as chief curator of architecture and design at the Museum of Modern Art (MoMA), where he played a pivotal role in the museum'sYoshio Taniguchi-designed expansion. Riley was brought on at MAM as the museum embarked on a $220 million new building project by starchitects Herzog & de Meuron, slated for completion in 2013; he'll now return to his own architectural practice, K/R, which he runs with John Keenen.

Today you announced that you are leaving the Miami Art Museum. What was the reason for your decision?
Recently a lot of things have come together. We finished the definitive design for the building, and it’s widely supported by the trustees and staff and has had great reception locally, among civic leaders and elected officials. We are on budget, our fundraising is going well, and we just finished the last of the major governmental approvals. In the last couple of weeks, I thought, I could either leave now or it will be another four years. This is the perfect moment to leave. The design is on target, and we have a good team in place in terms of the project managers.

I understand you are staying on with the museum initially, as a consultant.
I will stay involved, as a consultant and adviser to the building project, as long as it’s useful. John Keenen and I founded our architectural practice, Keenen/Riley — now called K/R — 25 years ago. When I left in 1991 to become a curator at MoMA, I expected that to be a five- or six-year hiatus, and now I find it being 18 years, so there’s a longing to go back to K/R. I love working with architects, but I’ve always missed working as an architect. Of course, we are in same position most architects are in these days, but we have significant projects in China, Mexico, and we are finishing a house in Sagaponeck, as part of a project organized by Richard Meier.

Did working on the new building for MAM make you especially wistful for your own architectural practice?
Yes. But what I’ve learned at MAM and MoMA is that architects can be really helpful to a museum in many different ways. I think my ability to think through architectural problems benefited those museums, and I don’t for a moment regret [taking those jobs]. It’s something only an architect can do for a museum. But of course I was constantly reminded of my own practice. There’s a reason they call it a profession; I feel professed. To go back to it was a quick decision once I made it. The chairman of the board at MAM, Aaron Podhurst, regretted my decision, but he generously offered the idea of my staying on as a consultant. I’ll still be a member of the team.

Given that MAM doesn’t have a large permanent collection, there has been some controversy over the new building. Did this have anything to do with your departure? How have MAM’s efforts to build up the collection been going?
We’ve been doing well with the permanent collection, and I’ve realized that building a collection is much more difficult than building a museum. While I feel I have a lot to give to an institution, and I have given a lot, when it comes to building a collection, it’s about more than simply romancing donors. It’s about having a thoughtful and strategic notion about which artworks are critical to conveying a certain message. That’s done by engaging collectors in a dialogue. It’s about language and conviction on the part of the director. I don’t feel like I’m beating myself up too much by saying that that is not the strongest thing I do. My background is in architecture and design. With my departure, there is an opportunity in terms of building the rest of the collection, by hiring staff — a director possibly — who’s capable of building a collection with same the conviction with which I have built the building project.