In this chapter, we covered some more theory about transactions and their ACID properties, isolation levels, and how PostgreSQL's isolation levels map to SQL standards. We covered the basics of PostgreSQL's MVCC. Along the way, we also had a look at a few system functions and columns that can be used both to understand the theory and to study system behavior in real life. In the next couple of chapters, we will focus on tools, one that can be used to design a database for PostgreSQL and another that can be used to work with the PostgreSQL cluster.