This book is a summary of many years of research into acts in proceedings. The complete account of the research results has been published in Polish as “Formalizacja i konwencjonalizacja jako instrumenty analizy czynności karnoprocesowych w prawie polskim” by the Adam Mickiewicz University Press. “Definitions and typologies…” discusses the issues of typology and definition of acts in criminal proceedings approached as formalised legally-relevant conventional acts. The monograph is underpinned by detailed methodological and theory-of-law premises, making use of the conceptions of conventional acts and constitutive rules as well as some components of the conception of performative utterances. The monograph adheres to the integrating model of law studies whereby Polish criminal procedural law is only a point of reference, while the research results are applicable to other legal systems, in particular written law ones.