BACKGROUND In recent years there has been a move towards a competency-based model for assessing the performance of practical procedures in clinical medicine rather than the traditional assumption that competency is achieved with increasing experience. For such an assessment to be valid, the necessary competencies comprising that skill must be identified. Our aim was to map the individual competencies necessary to perform a given procedural skill using spinal anaesthesia as the example, and to explore the relationship of individual competencies with each other. METHODS In the first part of the study an extensive hierarchical task analysis (HTA) was undertaken to determine the competencies necessary for the performance of spinal anaesthesia. Secondly, the concept of competency-based knowledge space theory (CbKST) was applied to the map. CbKST is based on the principle that acquisition of a specific skill is usually preceded by a number of dependent or prerequisite skills. Our aim was to map the individual competencies necessary to perform a given procedural skill RESULTS The analysis yielded a comprehensive HTA of the skills necessary to perform spinal anaesthesia, comprising 509 individual competencies. Applying the concept of CbKST yielded 194 key competences with at least one dependent or prerequisite skill. DISCUSSION We have defined a comprehensive HTA or competency map for use in the assessment of the performance of spinal anaesthesia. This CbKST approach will provide clinicians who undertake medical procedures to better understand their own performance, and to improve over time.