This monograph addresses the foundations of criminal liability in Polish law – act and guilt. Both act and guilt are elements of the structure of a criminal offence – a model that allows for the evaluation of human behaviour in terms of the possibility of incurring criminal liability. It is assumed that they are interconnected, constituting a sine qua non condition for punishment in a democratic state governed by the rule of law. The monograph addresses the essence of act and guilt, and one of its main research questions is whether, under Polish criminal law, the principles "no crime without act" and "no crime without guilt" apply without exception.