​The Engineering Society of Illinois (ESI), formerly Illinois Society of Professional Engineers (ISPE), established in 1886, is a professional association representing the interests of over 2,500 engineer members from all fields of practice. Headquartered in Springfield, ESI includes among its membership licensed professional engineers, licensed structural engineers, licensed land surveyors, engineer interns, graduate engineers and engineering students. 

ESI is governed by a Board of Directors made up of an elected Executive Committee and representatives appointed from each Chapter.

In addition to the main Society, ESI's affiliate organizations include the Illinois Engineering Initiative, a not-for-profit organization that provides scholarships to engineering students and funds the state MATHCOUNTS competition for junior high students, and the ESI PAC, a registered political action committee supporting Illinois candidates for elected office.

ESI is an organization that stands for the highest standards of honesty, integrity and ethical conduct, impartiality, fairness and equity, and a dedication to promoting public health, safety, and welfare.

ESI provides a voice for all licensed engineers by engaging in dialogue on engineering issues within Illinois State government, responding to legislation affecting the engineering profession and aggressively protecting the Professional Engineering Act in Illinois.