The 12th Brazilian Conference on Software: Theory and Practice (CBSoft 2021) occurred in the online edition, from September 27th to October 1st, 2021. The publication was coordinated by professor Cristiano Vasconcellos (UDESC) and Vanessa Collere (Univille), who organized the volumes together with the coordinators of each event. The Proceedings, which concentrate on the scientific papers approved in events of CBSoft 2021, are indexed below.

 Core Events:

• Proceedings of the 6th Brazilian Symposium on Systematic and Automated Software Testing (SAST)
• Proceedings of the 15th Brazilian Symposium on Software Components, Architectures, and Reuse (SBCARS)
• Proceedings of the 35th Brazilian Symposium on Software Engineering (SBES)
• Proceedings of the 25th Brazilian Symposium on Programming Languages (SBLP)
• Companion Proceedings of the 12th Congresso Brasileiro de Software: Teoria e Prática (CBSoft)

• Proceedings of the 1st Brazilian Workshop on Intelligent Software Engineering (ISE)
• Proceedings of the 3rd Workshop on Modeling and Simulation of Software-Intensive Systems (MSSiS)
• Proceedings of the 1st Workshop on Open Science Practices for Software Engineering (OpenScienSE)
• Proceedings of the 9th Workshop on Software Visualization, Evolution and Maintenance (VEM)