Recent security incidents and analysis have demonstrated that manual response to such attacks is no longer feasible. Intrusion Detection systems offer techniques for modelling and recognising normal and abusive system behaviour. Such methodologies include: statistical models, immune system approaches, protocol verification, file and taint checking, neural networks, whitelisting, expression matching, state transition analysis, dedicated languages, genetic algorithms and burglar alarms. This paper describes these techniques including an IDS architectural outline and an analysis of IDS probe techniques finishing with a summary of associated technologies.