  This paper is a contribution to the Computational Science & Engineering
Software Sustainability and Productivity Challenges (CSESSP Challenges)
Workshop (https://www.nitrd.gov/csessp/), sponsored by the Networking and
Information Technology Research and Development (NITRD) Software Design and
Productivity (SDP) Coordinating Group, held October 15th-16th 2015 in
Washington DC, USA. It introduces the role of software at the National Science
Foundation (NSF) and the NSF Software Infrastructure for Sustained Innovation
(SI2) program, then describes challenges that the SI2 program has identified,
including funding models, career paths, incentives, training, interdisciplinary
work, portability, and dissemination, as well as lesson that have been learned.
