The purpose of this thesis was to examine: 1. the diagnostic accuracy of routine clinical and saliva hydration markers to identify both intracellular and extracellular dehydration in an elderly hospitalised cohort; 2. the diagnostic accuracy of traditional and novel hydration marker to identify mild intracellular and extracellular dehydration in young healthy participants; 3. whether stimulating saliva improved the diagnostic accuracy and practicality of hydration assessment using saliva; 3. if mild intracellular and extracellular dehydration had a differential effect on endurance exercise performance and affective well-being. All routine physical signs, urine and saliva flow rate showed poor diagnostic accuracy for detecting either form of dehydration. In contrast, saliva osmolality demonstrated moderate diagnostic accuracy to distinguish both dehydration types, and may have utility for the assessment of both intracellular and extracellular dehydration in older individuals. Although stimulating saliva reduced collection time by 85%, stimulation reduced the diagnostic accuracy of both saliva flow rate and saliva osmolality to identify progressive mild to modest intracellular dehydration evoked by exercise and heat-stress. Diagnostic accuracy to identify mild intracellular dehydration was perfect for urine colour and specific gravity, near perfect for plasma osmolality, and fair for LF-HF, saliva osmolality and flow rate. Diagnostic accuracy to identify mild extracellular dehydration was generally poor, with the exception of postural heart rate change. A combination of thirst, urine colour and postural heart rate change improved dehydration diagnosis by discriminating between dehydration types. Endurance performance was worse after mild extracellular than intracellular dehydration. Markers of affective well-being were unaltered after extracellular dehydration, but cardiovascular and ventilatory strain appeared higher during exercise. In contrast, intracellular dehydration caused disruption to affective well-being, but did not alter cardiovascular and ventilatory parameters. Dehydration type appears to alter the magnitude and mechanism by which dehydration impairs endurance performance.