This article addresses the following questions: - Which changes in technique and which improvements in physical characteristics (such as flexibility, strength, endurance, …) result in increased performance? - What advice leads to these changes? and - What performance can finally be achieved ? Summary: In both genders, a similar pattern in the graphs connecting the scores for flexibility and strength at international level suggests a typical breaststroker`s profile. Combined m. latissimus-pectoralis and m. triceps strength correlate with breaststroke performance, notwithstanding large differences in style. In men, this specific strength and the flexibility in knee outward rotation and ankle dorsi-flexion are needed to produce effective screw-like sweeps, respectively with the hands and with the foot soles. Men, using on the average flatter styles than women, generate almost only screw-like propulsion. Less flexibility in knee outward rotation than in women could be compensated by a high score for hip outward rotation, resulting in a larger distance between the knees during spreading. In women, shoulder abduction correlates with performance. Moreover, women at international level score higher than those at lower levels for all body structure and composition variables. This specific shoulder abduction flexibility and body structure could be related to more marine animal-like propulsion with the participation of the whole body than in men. In a subsequent article, these propulsion principles will be addressed. All together, + 40% of the variables appear to be associated with the undulating style in women and with the flat style in men, which explains why many authors did not find relations with performance in the breaststroke. Using available tools in the field, an individual profile chart and a video-recording, an expert will soon be able to make a semi-automated qualitative diagnosis of club level swimmers. For higher-level swimmers an Evaluation Centre remains indispensable.