Software Sustainability.- Elements of Sustainability for Public Sector Software – Mosaic Enterprise Architecture, Macroservices, and Low-Code.- How could we have known? Anticipating Sustainability Effects of a Software Product.- Software Sustainability: Academic Understanding and Industry Perceptions.- Engineering and Management.- Tailored Design Thinking Approach - A Shortcut for Agile Teams.- Deliberative Technical Debt Management: An Action Research Study.- Blockchain Governance: A Dynamic View.- Framework for creating relevant, accessible, and adoptable KPI models in an Industrial Setting.- Risk exposure and management in software development – a survey of multiple software startups.- Software Startups.- Supporting Entrepreneurship with Hackathons - A Study on Startup Founders Attending Hackathons.- User Experience Practices in Early-Stage Software Startups - An Exploratory Study.- A Study of Factors on Women from the Tech Sector and Entrepreneurship.- MakingInternal Software Startups Work: How to Innovate Like a Venture Builder.- Software Platforms and Ecosystems.- The Economic Anatomy of Paid Crowdsourcing.- Power Relations within an Open Source Software Ecosystem.- When Player Communities Revolt Against the Developer: A Study of Pokémon GO and Diablo Immortal.- Continuous Improvement.- Design Principles for a Crowd-based Prototype Validation Platform.- What are critical success factors of DevOps projects? A systematic literature review.- Towards Federated Learning: A Case Study in the Telecommunication Domain.  

This book constitutes the refereed proceedings of the 21th International Conference on Software Business, ICSOB 2021, which was held during December 2-3, 2021. The conference was originally planned to take place in Drammen, Norway, but changed to an online format due to the COVID-19 pandemic. The special theme of ICSOB 2021 was software sustainability.The 13 full papers and 5 short papers presented were carefully reviewed and selected from 39 submissions. They deal with a range of topics including software sustainability, Agile development, DevOps, software startups, prototyping, software ecosystems, crowdsourcing platforms, technical debts, and risk management.