Full or Dual membership is open to any Head whose school meets our criteria. It is not uncommon (currently about a third of the members) for full members to be in membership of more than one Heads' Association. Those that are also members of HMC, GSA or ISA are known as Dual Members. Other categories of membership are open to schools which do not meet our criteria for full membership. Alliance Members are Heads of schools that, whilst not meeting our criteria for full membership, nevertheless fit well into our wider canon. Most of these are in the UK but some are overseas. We also retain as Honorary Members, a range of former members who have retired or moved on to other employment in the educational world.

A Head wishing to apply for membership in any category should contact the General Secretary, Clive Rickart, in the first instance. If appropriate, the General Secretary will make a preliminary visit to the school. The Head will be asked to submit an application form and supply documentation about the school, its results and its governance. If the details on the application form indicate that the school is likely to meet the criteria for membership, a visit from the General Secretary and a member of the Committee will be made. A report on the visit is submitted to the Committee. The final decision on any membership application rests with the Committee.