Bioelectronics for interrogation and intervention of cardiac systems is important for the study of cardiac health and disease. Interfacing cardiac systems by using conventional rigid bioelectronics is limited by the structural and mechanical disparities between rigid electronics and soft tissues as well as their limited performance. Recently, advances in soft electronics have led to the development of high-performance soft bioelectronics, which is flexible and stretchable, capable of interfacing with cardiac systems in ways not possible with conventional rigid bioelectronics. In this review, we first review the latest developments in building flexible and stretchable bioelectronics for the epicardial interface with the heart. Next, we introduce how stretchable bioelectronics can be integrated with cardiac catheters for a minimally invasive in vivo heart interface. Then, we highlight the recent progress in the design of soft bioelectronics as a new class of biomaterials for integration with different in vitro cardiac models. In particular, we highlight how these devices unlock opportunities to interrogate the cardiac activities in the cardiac patch and cardiac organoid models. Finally, we discuss future directions and opportunities using soft bioelectronics for the study of cardiac systems.