The Institution's main purpose, in accordance with its Charter, is to promote the general advancement of the science and art of structural engineering. As well as being an international learned society, it is also a qualifying body with high entry standards. The Institution has some 23,000 members, all of them as individuals. The Institution is not a trade association and firms and companies are not members. Furthermore, under its Charter the Institution cannot act as a mediation or an arbitration body.

There are local branches of the Institution throughout the United Kingdom and the Republic of Ireland; elsewhere members are organised into divisions where there are sufficient numbers.

Membership

The preparatory classes of membership are Student and Graduate.
Those who qualify as incorporated structural engineers are elected Associate-Members, with the designatory letters IEng AMIStructE.

Those with appropriate academic qualifications and experience are elected to corporate membership, as either Associates (AIStructE), Members (MIStructE) or Fellows (FIStructE). They are thereby chartered structural engineers, a description which is available only to corporate members of the Institution.

The Institution is always willing to state whether an individual is in membership. Some organisations use the Institution's logo to indicate that at least one partner or director is a member.

In the United Kingdom, however, there is no state registration of engineers, or restriction on the work undertaken by either members or non-members.

Professional Conduct

The Institution is a professional body and its Bye-Laws are approved by the U.K. Privy Council. The Code of Conduct laid down in its Bye-Laws states:

Every corporate member shall at all times so order his conduct as to uphold the dignity and reputation of his professional and to safeguard the public interest in matters of safety and health and otherwise. He shall exercise his professional skill and judgement to the best of his ability and discharge his professional responsibilities with integrity.

Every member of any class shall at all times so order his conduct as to uphold the dignity and reputation of the Institution and to act with fairness and integrity towards all persons with whom his work is connected and towards other members.

Every non-corporate member shall at all times exercise his skill and judgement and discharge his structural engineering responsibilities to the best of his ability.

The Council of the Institution has issues the following Rules of Conduct, which apply to all members: 1. A member of the Institution in his responsibility to the profession shall have full regard to the public interest. 2. A member of the Institution shall order his conduct so as to uphold the dignity, standing and reputation of the profession. 3. A member of the Institution shall discharge his duties with integrity. 4. A member of the Institution shall not maliciously or recklessly injure or attempt to injure, whether directly or indirectly, the professional reputation of another engineer. 5. A member of the Institution responsible for the control of a consulting practice shall ensure that its documentation properly describes its structure and organisation, for example as a sole practitioner, partnership, or private or public limited company.

These Rules have been amplified by Guidance Notes which are issued regularly to all members. The Rules deal with members' ethical behaviour. They do not relate to commercial matters such as fees, contract terms, copyright in drawings, or professional indemnity insurance.