Middlesex County Retirement System is a public pension fund based in Billerica, Massachusetts. Established in 1911, its a multi-employer defined benefit retirement plan for the members, employees, retirees, and beneficiaries of municipalities and other governmental subdivisions within Middlesex County. It oversees the management of the retirement system, invests the assets of the system, adopts actuarial assumptions necessary to properly fund the system, counsels members, and administers retirement, disability, and death benefits. The assets are managed by the board members.