JTorX: A tool for on-line model-driven test derivation and execution We introduce JTorX, a tool for model-driven test derivation and execution, based on the ioco theory. This theory, originally presented in [12], has been refined in [13] with test-cases that are input-enabled. For models with underspecified traces [3] introduced uioco. JTorX improves over its predecessor TorX [14] by using uioco and this newer ioco theory. By being much easier to deploy, due to improved installation, configuration and usage. And by integrating additional functionality, next to testing: checking for (u)ioco between models [6]; checking for underspecified traces in a model; interactive or guided simulation of a model. This makes JTorX an excellent vehicle for educational purposes in courses on model-based testing, as experience has shown – and its usefulness is not limited to education, as experience has shown too