[2] The mission of IDOT is to provide safe, cost-effective transportation for Illinois in ways that enhance quality of life, promote economic prosperity and demonstrate respect for the environment.

The department absorbed the functions of the former Department of Public Works and Buildings, acquired some planning and safety inspection functions of other state agencies, and received responsibility for state assistance to local mass transportation agencies such as the Chicago-area Regional Transportation Authority, which was in the process of being formed at this time.

On June 18, 2005, IDOT became the first state transportation agency to achieve ISO 9001:2000 certification for 23 key processes located in the Central Administrative Office and regional District Six.

On July 6, 2006, that certification was expanded to encompass all processes involved in the planning, design, and construction of road and bridge improvements, maintenance of roads and bridges, and administrative oversight in the Central Administrative Office and District Six.

In the spring of 2014, it was reported that IDOT had engaged in patronage hiring of numerous staff assistants – Ann Schneider resigning following this scandal.