Malnutrition is a liver cirrhosis complication affecting more than 20%-50% of patients. Although the term can refer to either nutrient deficiency or excess, it usually relates to undernutrition in cirrhosis settings. Frailty is defined as limited physical function due to muscle weakness, whereas sarcopenia is defined as muscle mass loss and an advanced malnutrition stage. The pathogenesis of malnutrition in liver cirrhosis is multifactorial, including decreased oral intake, maldigestion/malabsorption, physical inactivity, hyperammonemia, hypermetabolism, altered macronutrient metabolism and gut microbiome dysbiosis. Patients with chronic liver disease with a Body Mass Index of < 18.5 kg/m and/or decompensated cirrhosis or Child-Pugh class C are at the highest risk of malnutrition. For patients at risk of malnutrition, a detailed nutritional assessment is required, typically including a history and physical examination, laboratory testing, global assessment tools and body composition testing. The latter can be done using anthropometry, cross-sectional imaging including computed tomography or magnetic resonance, bioelectrical impedance analysis and dual-energy X-ray absorptiometry. A multidisciplinary team should screen for and treat malnutrition in patients with cirrhosis. Malnutrition and sarcopenia are associated with an increased risk of complications and a poor prognosis in patients with liver cirrhosis; thus, it is critical to diagnose these conditions early and initiate the appropriate nutritional therapy. In this review, we describe the prevalence and pathogenesis of malnutrition in liver cirrhosis patients and discuss the best diagnostic approach to nutritional assessment for them.