Diagnostics are necessary for sports achievements and the general progress of sports. In the focus of this paper are diagnostic procedures in the fitness preparation of football players. The basis for the implementation of diagnostics is on three phases of measurement during the training process, initial, transitive, and final, with the aim of controlling and determining the effects that can be achieved by programmed training. Diagnostic procedures help to assess the effectiveness of an earlier training procedure. In regular training practice, diagnostic data collected by various measuring tools is of great value. The main goal of this paper was to present the types of diagnostic procedures and the structure of fitness training. An additional goal was to emphasize the high importance of diagnostic procedures in the fitness preparation of football players. The paper is based on a systematic analysis of professional and research literature on diagnostic procedures and professional and research literature on the structure of fitness training. The paper presents a systematic overview of diagnostic procedures used for testing the fitness status of football players with a separate presentation of field tests and laboratory tests. Furthermore, the paper clarifies the importance of determining anthropometric abilities, morphological characteristics, and general health status of football players. Also, the structure of fitness training was explained through examples of both energy and neuromuscular training. There are a number of factors that influence the choice of diagnostic procedure of an individual fitness trainer. From a large database of tests, it is crucial to select sets of tests that provide a large amount of information about an individual athlete, in order to plan and determine the level of the aerobic and anaerobic energy capacity of athletes.