Kotlin is one of the newest languages in the Java ecosystem, eliminating many of the limitations of Java and being much more versatile. Among its advantages: full compatibility with Java and the ability to integrate at the source code level, broad support for the functional programming paradigm, which helps write reliable and secure code, concise syntax, and, most importantly, guarantees of support from the IT giant Google. Pierre-Yves Saumon, an experienced Java developer, in his book covers in detail the nuances of programming in Kotlin, moving from a general description of the language to its characteristic features and capabilities, including functional programming techniques. The publication is intended for developers familiar with Java and seeking to improve the security of their programs, as well as simplify their writing, testing and maintenance.