Architecture, both new and old, defines city skylines and has a lasting impact on our perceived memory of a place. And while historic architecture has its own charm, it’s no secret that, at its best, modern architecture has the ability to be inspiring. Examples abound, including almost any building designed by figures such as Zaha Hadid, Rem Koolhaas, Santiago Calatrava, and Frank Gehry, among others. Their buildings, much like a force of nature, have the ability to transform a neighborhood (almost always for the better). Many refer to this as the Bilbao effect, a term coined after a Frank Gehry–designed Guggenheim museum helped turn around Bilbao’s economy in Spain. Yet economy boost aside, what about when these modern marvels are built on or within the existing buildings themselves? While it’s not the norm, there are times when architects decide (mainly due to preservation), that instead of building around or in place of historic structures, it’s in fact better to build in or atop the original foundation. When these two worlds of old and new come together, the result can be awe-inspiring. From Zaha Hadid’s extension to the Port House in Belgium, which looks as if a spaceship were attached to a 19th-century building, to Daniel Libeskind’s beautiful clash of new and old with Canada’s Royal Ontario Museum, AD lists the eleven best examples of when modern and historic architecture come together to produce something better than the sum of their parts.