Machine perfusion has gained rapid clinical utility and is now a critical component of liver transplantation. Perfusion may either be conducted ex situ, after liver procurement, or in situ, as a component of procurement. There are two major strategies of ex situ perfusion at the present time: Hypothermic-oxygenated perfusion (HOPE) and Normothermic Machine Perfusion (NMP). Both are currently practiced in Europe, though only NMP is approved in the United States. Both approaches offer critical logistical benefits without harming the liver graft, and both have been linked to improved graft utilization in multiple allocation systems. HOPE offers benefits in protecting the graft from ischemic-reperfusion injury (IRI) by reducing mitochondrial injury and downstream inflammation, which can lessen the rate of ischemic cholangiopathy (IC). However, HOPE may not offer the same breadth of available markers for viability assessment compared to NMP, though increasing research is narrowing this gap. Mitochondrial injury can be quantified through flavin-mononucleotide (FMN) release from complex I at reperfusion under cold conditions. NMP-perfused grafts are metabolically active, which may facilitate viability testing, yet the reintroduction of oxygen at physiologic temperatures seems to have, at best, no protective effect on the liver and particularly the biliary tree. In situ perfusion consists primarily of normothermic regional perfusion (NRP), which is commonly employed for donation after cardiac death (DCD) grafts. Ongoing research is critical to establishing which approach is right for each graft, though both may be confidently used in current practice.