Digital Twins are an emerging concept which is gaining importance in several fields. It refers to a comprehensive software representation of an actual system, which includes structures, properties, conditions, behaviours, history and possible futures of that system through models and data to be continuously synchronized. Digital Twins can be built for different purposes, such as for the design, development, analysis, simulation, and operations of non-digital systems in order to understand, monitor, and/or optimize the actual system. To realize Digital Twins, data and models originated from diverse engineering disciplines have to be integrated, synchronized, and managed to leverage the benefits provided by software (digital) technologies. However, properly arranging the different models, data sources, and their relations to engineer Digital Twins is challenging. We, therefore, propose a conceptual modeling framework for Digital Twins that captures the combined usage of heterogeneous models and their respective evolving data for the twin's entire life-cycle.