The Jožef Stefan Institute (JSI) (Slovene: Institut »Jožef Stefan«, IJS) is the largest research institute in Slovenia. The main research areas are physics, chemistry, molecular biology, biotechnology, information technologies, reactor physics, energy and environment. At the beginning of 2013 the institute had 962 employees, of whom 404 were PhD scientists.

The mission of the Jožef Stefan Institute is the accumulation and dissemination of knowledge at the frontiers of natural science and technology for the benefit of society at large through the pursuit of education, learning, research, and development of high technology at the highest international levels of excellence.

The institute was founded by the State Security Administration (Yugoslavia) in 1949 for atomic weapons research. Initially, the Vinča Nuclear Institute in Belgrade was established in 1948, followed by the Ruđer Bošković Institute in Zagreb in 1950 and the Jožef Stefan Institute as an Institute for Physics in the Slovenian Academy of Sciences and Arts. It is named after the distinguished 19th century physicist Josef Stefan (Slovene: Jožef Stefan), best known for his work on the Stefan–Boltzmann law of black-body radiation.

IJS is involved in a wide variety of fields of scientific and economic interest. After close to 60 years of scientific achievement, the institute has become part of the image of Slovenia.

Over the last 60 years it has created a number of important institutions, such as the University of Nova Gorica, the Jožef Stefan International Postgraduate School and the Ljubljana Technology park.

• Physics
  • F1 Theoretical Physics
  • F2 Low and Medium Energy Physics Archived 2016-03-07 at the Wayback Machine
  • F3 Thin Films and Surfaces
  • F4 Surface Engineering and Optoelectronics
  • F5 Condensed Matter Physics
  • F7 Complex Matter
  • F8 Reactor Physics
  • F9 Experimental Particle Physics at the Wayback Machine (archived August 6, 2007)
• Chemistry and Biochemistry
  • B1 Biochemistry and Molecular Biology
  • B2 Molecular and Biomedical Sciences
  • B3 Biotechnology
  • K1 Inorganic Chemistry and Technology Archived 2012-02-19 at the Wayback Machine
  • K3 Physical and Organic Chemistry
  • K5 Electronic Ceramics
  • K6 Engineering Ceramics
  • K7 Nanostructured Materials
  • K8 Synthesis of Materials K8
  • K9 Advanced Materials
  • O2 Environmental Sciences
• Electronics and Information Technologies
  • E1 Automation, Biocybernetics and Robotics
  • E2 Systems and Control
  • E3 Artificial Intelligence
  • E5 Open Computer Systems and Networks
  • E6 Communication Systems
  • E7 Computer Systems
  • E8 Knowledge Technologies
  • E9 Intelligent Systems
• Reactor Engineering and Energetics
  • R4 Reactor Engineering
• Centers
  • Centre for Knowledge Transfer in Information Technologies
  • Centre for Electron Microscopy at the Wayback Machine (archived August 12, 2011)
  • Energy Efficiency Centre at the Wayback Machine (archived October 2, 2011)
  • Networking Infrastructure Centre
  • Milan Čopič Nuclear Training Centre at archive.today (archived April 15, 2013)
  • Reactor Infrastructure Centre
  • Scientific Information Centre Library
  • Center for Technology Transfer and Innovation

The institute has facilities in two locations. The main facilities and the headquarters are on Jamova 39 in Ljubljana, the other location is the institute's Podgorica Reactor Center located in Brinje, Dol pri Ljubljani near Ljubljana.

The institute's Podgorica Reactor Center is home to a pool type research reactor. The General Atomics TRIGA Mark II reactor is rated for a nominal 250 kW thermal. The reactor was first licensed in 1966 and is expected to continue operation at least into the 2030s.

The Central Radioactive Waste Storage of Slovenia is co-located at the institute's reactor facility. This facility is used for storage of the low and intermediate level solid radioactive waste from the Podgorica Reactor Center and other, non-institute small waste producers such as medical, research, and industrial applications of ionising radiation.

• IST-World Portal for analysis of European research at the Wayback Machine (archived February 8, 2007)
• VideoLectures.Net Collection of videos of scientific events
• AnswerArt Question Answering system
• SearchPoint Contextual Web Search

• Official website

46°2′32.29″N 14°29′15.97″E / 46.0423028°N 14.4877694°E / 46.0423028; 14.4877694