The new training concept CrossFit® is supposed to build comprehensive fitness and is already applied by several professionals that depend on multisided, functional physical skills, such as police, firefighters, and military units. Also, as a leisure sport, the CrossFit® program quickly developed into a fast-growing and successful brand with approximately 15,000 affiliated training centers worldwide. Thereby, the basic principles are still poorly understood. The training routine involves constantly varied functional movements performed at high-intensity and includes exercises from the main elements of gymnastics (e.g., pull-ups, push-ups, and burpees), weightlifting (e.g., powerlifting, and Olympic weightlifting), and cardiovascular activities (e.g., running, rowing, and jumping), usually referred to as 'Workout of the Day'. Motivated by the intense community character of CrossFit® training, individuals, regardless of whether they are members of special forces, professional athletes, patients with chronic illnesses, disabled, overweight, or untrained people, gladly face the daily workouts and perform them together in the same training sessions due to the scalability of exercises. Thus, the concept of CrossFit® training works effectively, although it has not yet been scientifically evaluated or understood in detail why or how. Characteristic differences from other sports, such as the constant variation of the training stimuli, the unpredictability of the competitions, and psycho-social aspects of the community, require a detailed scientific analysis of CrossFit® in order to provide practical recommendations for athletes and coaches. For this reason, a number of research questions were considered in parallel and summarized in this thesis with the overarching aim of providing a detailed insight into the nature of CrossFit®. In this context, the focus of this thesis is on (a) investigating the acute, short-term physiological and cardiovascular responses of CrossFit® training, (b) determining a CrossFit® performance profile and assessing the predictors of competitive performance, and (c) examining the impact of the consequences of the COVID-19 pandemic on training behavior of CrossFit® athletes and describing the way the athletes dealt with the new situation. Within this thesis, two intervention studies and two questionnaire surveys were conceived to address these issues. In detail, the experimental analysis of physiological parameters (rating of perceived exertion, blood lactate, and heart rate [HR] values) of an ultra-short, intense CrossFit® workout was intended to provide whether short CrossFit® workouts (< 2 min) lead to an increase in blood lactate concentration after the completion of the workout and whether the rating of perceived exertion is related to the increase in lactate concentration. Within this pilot study, for the first time, a time-delayed increase in blood lactate values was observed, as is known from short runs, although not from CrossFit® workouts. In a further study, cardiovascular demands were analyzed in four different training sessions at a local training center to assess physiological responses not on isolated workouts separately, even during 1-h sessions. In this regard, the observational study of athletes of different experience levels provides an analysis of practical CrossFit® training settings. The data suggest that the previous assumption that CrossFit® training is performed predominantly in the HR range above 90% of HRmax is misunderstood. Rather, the results suggest that the CrossFit® training provides a progressive cardiovascular load increase during 1-h training sessions and that beginner and experienced athletes, regardless of individual CrossFit® experience, push themselves to the limit without significant differences in cardiovascular response to the training stimulus. Given the lack of valid data in the scientific literature on performance parameters of CrossFit® athletes and the non-existing knowledge of regional differences, another research project was conceived to target this issue. For this reason, data from American and German athletes were collected by a questionnaire survey to determine a CrossFit® Benchmark performance profile. The results demonstrate no overall performance difference between the nations and suggest an important role of the back squat performance in the assessment of the physical fitness of CrossFit® athletes. Furthermore, the ongoing course of the COVID-19 pandemic also poses unprecedented challenges. For this reason, the following research investigated the use of digital sports offers, training habits, body weight changes, and purchase of sports equipment during the first SARS-CoV-2 lockdown in Germany via an online questionnaire. As a result, the data show that CrossFit® athletes purchased new equipment for a home gym and the use of digital sports increased significantly across all age groups. Despite the massive restrictions, the athletes were able to continue their training, and thereby, a subgroup reported a significant reduction in body weight, which may lead to an improvement in body composition and health-related aspects. In summary, the studies provide new insights into the physiological parameters of CrossFit® training, which constitute the foundation for prospective, controlled long-term trials. Based on the results, future research may investigate the performance- and health-promoting effects of CrossFit® in more detail. Nevertheless, this thesis contributes to major challenges in CrossFit® science by improving the comprehension of physiological demands, performance assessment, and training behavior of CrossFit® athletes, even during challenging times.