**1. Introduction**

The urgent need to reduce greenhouse gases and toxic emissions is fostering a rapid shift towards more sustainable transport. In this respect, hybrid electric vehicles (HEVs) emerge as a sustainable solution to increasingly adopt on a large scale very shortly. In HEVs, the internal combustion engine (ICE) efficiency is significantly improved by cooperating with an electric unit, consisting of one or more electric machines and a battery pack, that supports the engine during transient operations and enables regenerative braking. As a result, a significant reduction in fuel consumption and emissions can be achieved while maintaining high vehicle performance and driving comfort. A wide variety of hybrid electric powertrains is currently available on the market, ranging from various degrees of electrification, i.e., mild, full, and plug-in hybrid, and different driveline architectures, i.e., series, parallel, and power-split hybrid [1]. The main challenges of HEVs lie in the powertrain design, not only in terms of thermal, mechanical, and electrical components but also of energy management strategy. Consequently, effective research efforts in this field should integrate contributions and knowledge from several scientific areas, such as mechanical, electrical, controls, chemical, and energetic. In this respect, the Special Issue succeeded in collecting nine research articles coming from the sectors mentioned above, providing a broad perspective on open challenges for HEVs.
