Importance measures have been widely used as important decision-aiding indicators in various purposes such as reliability studies, risk analyses and maintenance optimization. In this work, we propose a novel time-dependent importance measure for systems composed of multiple non repairable components. The proposed importance measure of a component/group of components is defined as its ability to improve the system reliability during a mission given the current conditions (states or degradation levels) of its components. To take into account economic aspects (e.g., maintenance costs, economic dependence between components and the cost benefit thanks to maintenance operations), an extension of the proposed importance measure is then investigated. Thanks to these proposed importance measures, the component/group of components can be “optimally” selected for preventive maintenance regarding to the reliability criteria and/or the financial issues. A numerical example is introduced to illustrate the use and the advantages of the proposed importance measures.