Patrick G. Ryan announced late last month that he would step down as chief executive officer of Chicago-based Aon Corp. after running the company and its predecessor for four decades. At the same time, Michael D. O'Halleran announced that he is not a candidate for the CEO position and said he would give up his role as Aon's president and chief operating officer.

Mr. Ryan will continue as Aon's chairman, and the company was to conduct a search for a new president and CEO. Following are some of the notable events in Mr. Ryan's tenure, during which he was the architect of the world's second-largest brokerage.

1964: Mr. Ryan, 26, creates Ryan Insurance Group.

1978: Ryan Insurance acquires four brokers from Chicago-based Esmark and moves to No. 19 in Business Insurance's ranking of the largest U.S. brokers.

1981: Ryan Insurance acquires James S. Kemper & Co.

1982: Combined International Corp. acquires Ryan Insurance Group; Mr. Ryan is named president and CEO of Combined. Four months later, Combined buys Rollins Burdick Hunter Co. and becomes Rollins Burdick Hunter Group Inc.

1989: Rollins Burdick Hunter acquires Bayly, Martin & Fay International Inc.

1991: Rollins Burdick Hunter acquires Hudig-Langeveldt Group B.V.

1992: Rollins Burdick Hunter acquires Frank B. Hall & Co. Inc., becomes Rollins Hudig Hall Group Inc.

1995: Rollins Hudig Hall changes name to Aon Group Inc.

1995: Aon becomes world's second-largest broker in BI ranking.

1996-1997: Aon acquires Bain Hogg Group P.L.C., Alexander & Alexander Services Inc., Minet Group Inc., Sodarcan Inc. and Jauch & Huebener KGaA.

2000: Aon announces business transformation plan intended to streamline operations.

2001: Aon becomes world's largest captive manager with acquisition of SINSER Holding A.B. from Skandia Group.

2001: Aon loses 175 employees located in the World Trade Center in Sept. 11 attacks.

2003: Aon becomes first foreign broker licensed in China and India.

2004: Mr. Ryan announces he will step down as Aon CEO.