Hybrid Systems-in-Foil (HySiF) is a concept that extends the potential of conventional More-than-More Systems-in/on-Package (SiPs and SoPs) to the flexible electronics world. In HySiF, an economical implementation of flexible electronic systems is possible by integrating a minimum number of embedded silicon chips and a maximum number of on-foil components. Here, the complementary characteristics of CMOS SoCs and larger area organic and printed electronics are combined in a HySiF-compatible polymeric substrate. Within the HySiF scope, the fabrication process steps and the integration design rules with all the accompanying boundary conditions concerning material compatibility, surface properties, and thermal budget, are defined. This Element serves as an introduction to the HySiF concept. A summary of recent ultra-thin chip fabrication and flexible packaging techniques is provided. Several bendable electronic components are presented demonstrating the benefits of HySiF. Finally, prototypes of flexible wireless sensor systems that adopt the HySiF concept are demonstrated.