PDDL2.1 is an extension of the Planning Domain Description Language (PDDL) designed to express temporal planning domains. It was developed to address the challenges of modeling time and numeric resources in planning problems. The language was used in the third International Planning Competition (IPC-3) to enable planners to handle time and numeric constraints. PDDL2.1 extends PDDL with features such as numeric expressions, durative actions, and plan metrics, allowing for more expressive and realistic planning domains. It is backward compatible with the original PDDL, ensuring that existing domains and plans remain valid. The language supports both discretized and continuous durative actions, enabling the modeling of temporal properties. Plan metrics allow for the evaluation of plans based on numeric values, such as energy consumption. The semantics of PDDL2.1 ensure that plans are validated correctly, even when concurrent activities are involved. The paper discusses the syntax, semantics, and modeling capabilities of PDDL2.1, as well as the challenges and implications of using plan metrics and durative actions in planning. It also highlights the importance of ensuring that planning problems are well-defined and that the use of metrics does not lead to undecidable problems. The paper concludes that PDDL2.1 provides a powerful framework for modeling complex planning domains and that further research is needed to fully exploit its capabilities.PDDL2.1 is an extension of the Planning Domain Description Language (PDDL) designed to express temporal planning domains. It was developed to address the challenges of modeling time and numeric resources in planning problems. The language was used in the third International Planning Competition (IPC-3) to enable planners to handle time and numeric constraints. PDDL2.1 extends PDDL with features such as numeric expressions, durative actions, and plan metrics, allowing for more expressive and realistic planning domains. It is backward compatible with the original PDDL, ensuring that existing domains and plans remain valid. The language supports both discretized and continuous durative actions, enabling the modeling of temporal properties. Plan metrics allow for the evaluation of plans based on numeric values, such as energy consumption. The semantics of PDDL2.1 ensure that plans are validated correctly, even when concurrent activities are involved. The paper discusses the syntax, semantics, and modeling capabilities of PDDL2.1, as well as the challenges and implications of using plan metrics and durative actions in planning. It also highlights the importance of ensuring that planning problems are well-defined and that the use of metrics does not lead to undecidable problems. The paper concludes that PDDL2.1 provides a powerful framework for modeling complex planning domains and that further research is needed to fully exploit its capabilities.