This set of guidelines looks into the assistance and support given to those suffering from trauma and the treatment they recieve. Over the last two decades research has established a substantive evidence base in relation to trauma. However a huge gap still exists between evidence about the effects of trauma on individuals and possibilities for recovery, as well as in the treatment and service responses which enable sustained recovery. The ASCA Practice Guidelines for Treatment of Complex Trauma and Trauma Informed Care and Service Delivery (the Guidelines) are informed by this research. They fill the long overdue gap in knowledge, understanding and practice. Trauma is often solely characterised as a one-off event. Yet repeated extreme interpersonal trauma resulting from adverse childhood events (`complex’ trauma) is not only more common, but far more prevalent than currently acknowledged, including within the mental health sector. The effects of complex (cumulative, underlying) trauma are pervasive, and if unresolved, negatively impact mental and physical health across the lifespan.