When students experiences difficulties in mathematics, they require assistance to overcome the misconceptions or inappropriate ways of thinking they have developed. Such difficulties should be identified early and assistance provided to address the needs revealed by a Diagnosis of their ways of acting and thinking. Intervention can then focus on providing support within a teaching sequence as soon as a difficulty is noted or anticipated as an essential element of ongoing learning. In this way, students be can helped to develop the conceptual understanding and fluent processes needed to acquire and use mathematics. This paper will provide an overview to the intervention process, from determining underlying causes of any difficulties, leading a student to see inadequacies in their ways of proceeding and thus appreciate a need to change, to implementing means of building appropriate ways of thinking, generalising and applying mathematical ideas. A series of numeracy screening tests (Booker 2011) will be presented.