This paper introduces the developments of the functional model of legal capacity in the Common Law tradition to Spanish-speaking academic audiences. To achieve this, a brief comparison is drawn between different models to assess whether an adult lacks the necessary capacity to enter legal transactions. It is observed that, out of all these models, the functional model is the only one currently enjoying relative acceptance. For this reason, we comment on its virtues and defects. After the introductory part, this piece moves on to a study of how forensic practice in three jurisdictions that are considered examples of this legal tradition, namely England and Wales in the United Kingdom, British Columbia in Canada, and Queensland in Australia, all regulate the legal capacity of people with disabilities.