Testing is crucial in Kotlin Multiplatform (KMP) to ensure reliability across platforms. This codelab explores Kotest, a powerful testing framework for Kotlin, covering test source sets (commonTest, jvmTest, iosTest) and Kotest’s integration with JUnit (JVM) and Mocha (JS). You'll set up Kotest in a KMP and Compose Multiplatform (CMP) project, run tests across platforms, and learn different test styles (FunSpec, DescribeSpec, etc.). Through hands-on exercises, you'll write unit tests using Kotest matchers, implement lifecycle hooks, explore property-based testing, configure test parallelism and timeouts, and test coroutines and concurrency. By the end, you'll have a solid foundation in multiplatform testing with Kotest to build robust, scalable applications.