When and how to intervene in families where there are concerns about child maltreatment are dilemmas which professionals continually face. In England and Wales ‘significant harm’ is the threshold for compulsory state intervention into private family life that can lead to the permanent removal of children. The legislation does not include a clear operational definition of ‘significant harm’ and this is both a strength and weakness. This chapter explores how the concept is being interpreted and applied in practice. Themes discussed include the concept of ‘significance’, interpretations of ‘harm’ in an unequal society and the influence of the wider contexts.