In the past two decades, trade unions have sought to counter membership decline, initially through a focus on organizing. More recently the union renewal project has prompted a re-examination of whether unions can use different types of inter-union collaboration to build power for workers and the movement. This article examines two examples of such inter-union collaboration, the Transport Unions Federation and the Australian Workers’ Union–Maritime Union of Australia Offshore Alliance. The development of these organizational relationships is important to the trade union movement in Australia and internationally as examples of new forms of external solidarity.