Marine hydrokinetic (MHK ) energy is clean, renewable, and available worldwide. It comes in two forms: vertical in waves and horizontal in currents, tides, and rivers. Apart from a few major ocean currents, most of the ocean currents have flow speeds less than 3 kn and most rivers have speeds less than 2 kn, making harvesting of their MHK energy by steady-lift technologies (turbines) challenging. Horizontal MHK energy can also be harnessed using alternating-lift technologies (ALT s). Fish utilize alternating lift to propel efficiently in water either as individuals or in schools. Engineered structures – bluff bodies, such as circular cylinders and prisms, or slender bodies like hydrofoils – may generate alternating lift in quasi-steady uniform flows. When these structures have scale-relevant flexibility, severe flow–structure interaction (FSI ) phenomena may be induced. In typical engineering applications, FSI phenomena are destructive and, thus, avoided by design or suppressed using excessive damping or appendages. If FSI are instead enhanced, they may result in vigorous flow-induced motion (FIM ) of the body, leading to the conversion of MHK energy to potential and kinetic energy in a mechanical oscillator. Hydrofoils can harvest MHK energy through flutter – a well-studied and understood form of instability. On the other hand, bluff bodies, such as circular or rectangular cross-section cylinders, may exhibit several forms of FIM, individually or in schools that have been studied extensively but still are not well understood for either suppression or enhancement. Those FIMs are vortex-induced vibration (VIV ), galloping, buffeting, and gap flow in multibody interactions. When enhanced, they convert MHK energy to mechanical energy with high-power density (power-to-weight ratio) even from low-speed horizontal flows. This chapter presents an overview of the concepts of ALTs, the underlying physical principles, the available experimental and computational methods for studying the relevant FIM, the research challenges that have been overcome and those lying ahead, field-deployment progress, technology development, and bench marking.