More than 200 years ago lactate and oxygen had been identified as important chemical components and shortly later they were linked with metabolism and physical activity. Compared to these roots, threshold concepts as measures of aerobic fitness are relative recent developments invented during the second half of the last century. All thresholds are based on specific patterns of individual or combined increases in measures such as lactate, oxygen uptake, carbon dioxide production and ventilation during exercise tests with an increasing workload. Thresholds are supposed to identify objectively the transition from one to another metabolic state related to aerobic and anaerobic components of energy metabolism. They are useful measures of aerobic fitness and may monitor selected adaptations to training more sensitively than the maximum oxygen uptake. However, different threshold concepts and testing procedures provide different estimates of aerobic fitness, which are not directly comparable. The maximal lactate steady state is no unique threshold concept. It shall indicate the highest prolonged constant workload that can be sustained without anaerobic energy. At higher exercise intensity some anaerobic energy is required as indicated by a continuous increase in lactate over time whilst the workload is kept constant. The maximal lactate steady state has been used to validate selected threshold concepts. Workloads detected by most threshold concepts and the maximal lactate steady state are equally well correlated with best performances in testing and competition, and with multiple exercise intensities predicted or observed in prospective or observatory training studies. However, there is no evidence that any specific threshold or the maximal lactate steady state directly predicts an optimal training load. There is a vital need for multi-disciplinary integrative and applied research to close the increasing gap between the rapid growth in the understanding of factors and mechanisms regulating and limiting metabolism on cellular and molecular level and the corresponding complex interplay of effects on whole body performance and non- and minimal invasive measures of physiological acute responses.