From smart buildings to medical devices to smart nations, software systems increasingly integrate computation, networking, and interaction with the physical environment. These systems are known as Cyber-Physical Systems (CPS). While these systems open new opportunities to deliver improved quality of life for people and reinvigorate computing, their engineering is a difficult problem given the level of heterogeneity and dynamism they exhibit. While progress has been made, we argue that complexity is now at a level such that existing approaches need a major re-think to define principles and associated techniques for CPS. In this paper, we identify research challenges when modelling, analysing and engineering CPS. We focus on three key topics: theoretical foundations of CPS, adaptive methods for CPS, and exemplars of CPS serving as a research vehicle shared by a larger community. For each topic, we present an overview and suggest future research directions, thereby focusing on selected challenges. This paper is one of the results of the Shonan Seminar 118 on Modelling and Analysing Resilient Cyber-Physical Systems, which took place in December 2018.