Charles Jencks, whose writing on architecture helped define the field after Modernism, and who put his ideas into practice both in memorable landscape architecture and in overseeing the creation of Maggie’s Centers , buildings specifically designed for cancer patients, died on Sunday at his home in London. He was 80.

His sister , Penelope Jencks, said the cause was cancer.

Mr. Jencks was an architectural historian who, with a landmark book, put himself at the forefront of the debate over what architecture should do.

“Charles boldly announced the death of Modernism in his 1977 book ‘The Language of Post-Modern Architecture,’” Joseph Giovannini , who writes frequently about architecture, said by email. “He also brashly offered its replacement, Post-Modernism , stirring angry style wars that raged in architecture for more than a decade.”

Mr. Jencks had little use for Modernism, the style emphasizing geometric forms and rational use of space and eschewing ornamentation, which had dominated the first three-quarters of the 20th century. Instead he advocated “radical eclecticism” — he believed architecture should reflect the environment, should embrace symbolism and metaphor, and should celebrate distinct styles and merge different influences.