The quantification and monitoring of training load have been the topic of many scientific works in the last fifteen years. Training load monitoring helps coaches to individually prescribe, follow-up, analyze, adjust and program training sessions. In particular, the aim of the present book is to provide a critical literature report regarding different markers of training load monitoring, particularly in soccer, as the load is specific to individual sports. Therefore, the interests and limitations of different objective internal markers like heart rate, heart rate variability and biochemical variables (blood, urinary and hormonal variations); objective external markers like physical activity profiles and some physical tests; and subjective markers like the rate of perceived exertion, wellness index, the profile of mood states, the recovery-stress questionnaire for athlete and the daily analysis of life demands for athletes questionnaire are analysed, with a special focus on daily measures (before, during and after training) and monitoring throughout a whole season. This book aims at spreading the current scientific knowledge to any passionate and/or practitioners on the field.