The need to quantify aspects of training to improve training prescription has been the holy grail of sport scientists and coaches for many years. Recently, there has been an increase in scientific interest, possibly due to technological advancements and better equipment to quantify training activities. Over the last few years there has been an increase in the number of studies assessing training load in various athletic cohorts with a bias toward subjective reports and/or quantifications of external load. There is an evident lack of extensive longitudinal studies employing objective internal-load measurements, possibly due to the costeffectiveness and invasiveness of measures necessary to quantify objective internal loads. Advances in technology might help in developing better wearable tools able to ease the difficulties and costs associated with conducting longitudinal observational studies in athletic cohorts and possibly provide better information on the biological implications of specific external-load patterns. Considering the recent technological developments for monitoring training load and the extensive use of various tools for research and applied work, the aim of this work was to review applications, challenges, and opportunities of various wearable technologies.Keywords: internal load, training technology, wearable technology, external load, GPS Training-load monitoring has recently gained momentum in sport science, possibly due to technological advancements and better equipment to quantify training activities. 1 The reason for such interest resides in the need to improve and individualize the design of training and exercise programs to maximize the improvements in athletic performance and avoid overtraining and overreaching. Training prescriptions have been notionally based on the concept of progressive overload since humans embarked in structured sport and physical activity. Early examples of training prescription 2 gave clear indications that a scientific approach to training was important not only to identify appropriate progression strategies 3 but also to individualize the training dose and maximize performance. Training activities and/or exercise programs are designed with the aim of producing stimuli capable of triggering various physiological responses leading to improvements in the form and function of various biological systems. Early research by Selye 4 on stress shaped the thinking of modern approaches to training and exercise prescription 5 and laid the foundations for a systematic approach to quantify and describe adaptive responses to various exercise and training paradigms. It is a well-accepted notion that training activities can alter homeostasis and affect various physiological structures which respond to the training "stress" by trying to restore homeostasis. The net result of a well-designed progressive training program is an improvement in the structure and function of the target physiological systems which leads to improvements in human performance. However, the outcome of a po...