This chapter analyses cyber-physical systems (CPSs) under cyber-attacks. It first specifies the configuration and models of the CPS under study, then examines the impact of integrity attacks, particularly their potential to impair system dynamics. The key finding is that cyber-attacks affect only the plant image subspace: additive attacks act like unknown inputs added to the reference signal, and multiplicative attacks appear as uncertainty in the feedback controller. From this, two conclusions follow: (i) Detection and control of CPSs under cyber-attacks and under process faults are dual problems, and (ii) the real challenge lies in detecting and mitigating uncertainties in the plant image subspace. The chapter also analyses the role of the input residual signal and formulates problems for subsequent work.