How do ISO and six sigma fit together

The recent versions of ISO 9000 standards do incorporate the six sigma methodology in a section appropriately named Continual Improvement.

ISO 9004 gives guidance on a wider range of objectives of a quality management system than does ISO 9001,particularly for the continual improvement of an organization’s overall performance and efficiency, as well as its effectiveness.

I would recommend ISO 9004 as a guide for organizations whose top management wishes to move beyond the requirements of ISO 9001, in pursuit of continual improvement of performance.