The National Academy of Sciences called of the XL comprises two Classes of Members: National Members and Foreign Members.
The class of National Members consists of 40 Ordinary Members and a variable number of Supernumerary ones, determined by the number of Ordinary Members who are 70 years old and have at least 5 years of seniority in the appointment.
The division into Ordinary and Supernumerary members is a purely historical one and is linked to the order in which members succeeded the 40 Academy founders.
The Class of Foreign Members comprises 25 scientists.