Beginning in the 1990s, until his death in 2010, Kheel devoted himself to addressing the fundamental conflict between development and the environment. Kheel not only provided leadership to NNF after its founding in 1991, but contributed to the charity in other ways as well, donating most of his free time and significant personal financial resources to it. Kheel's historic archives are available to the public at the Kheel Center at Cornell's School of Industrial and Labor Relations, where further information may be found on his career and the disputes he helped resolve. He summarized his techniques and thinking on conflict resolution in general in his book, the Keys to Conflict Resolution. He was founder, director, and president of several other nonprofits with a mission closely related to that of NNF.