Aim: The purpose of this study is to present some general questions and problems of data gethering in criminal cases in aspect of criminal procedure law and forensic sciences. Methodology: Presentation, analysis, evaulation and drawing conclusions of legal rules, studies, papers, and textbooks in the subject area, both in Hungarian and Anglo-Saxon literature. Findings: Data gathering is a well-known method in both forensic sciences and criminal procedure law. However its meaning is different in these two fields of criminal sciences. Before the current code of criminal procedure in Hungary the meaning and content of data-gathering was similar in forensic science and in legal rules. But the new code has integrated the instruments of crime detection purposed secret information-gathering appliapble before the starting of investigation, and made them criminal procedural instruments under the name of covert instruments. Besides that the new code significantly changed the system and legal rules of „open” data-gathering methods. Nowdays some data-gathering methods only aply as covert instruments with all its strict conditions, consequences and formal rules. In my study first I try to withdraw the conceptual borders of data-gathering and outlines some historical backgrounds. After it I explain the conception and content of data gathering in criminalistic literature, then I examine the rules of data gathering in criminal procedure law. In the end of my paper I summarize the problems of this conceptual dualism. Value: The (open) data gathering in criminal cases as a subject has not been examined recently, so this study can draw attention on the problems of the latest legal regulation and the extension and modernization of criminalistic recomendations.