## 1 Introduction  
Food supply chains have a vast complexity which is often the cause of lack of transparency and traceability. On top of that, a major issue directly affecting public health is food safety. During the past twenty years, various food epidemic incidents have been reported, like the foot-and-mouth Disease in Europe in 2001, the _Escherichia coli_ outbreak in spinach in 2006 in USA, the Sanlu milk scandal in China in 2008, the _E. coli_ O104:H4 outbreak in Germany in 2011, the South African listeriosis outbreak in 2017-2018, etc. Governments and health organizations, in an attempt to prevent such dangerous outbreaks, have established relevant directives, laws as well as standards and regulations. For example, in Europe, the traceability of food products is compulsory according to the European Directive 178/2002 since 1 January 2005, together in compliance with the HACCP (Hazard Analysis and Critical Control Points) principles. Likewise, regulations all over the world have been established aiming to diminish food epidemic incidents.  
Nowadays, consumers' concerns regarding food provenance and quality are extremely high, resulting in the tendency to spend more money on food products whose origin is certified. Despite the developed technologies that are already in use, in many cases, the vast majority of the traceability systems are centralized, asymmetric and outdated in terms of data sharing and interoperability. Existing systems lack in transparency and consumers' trust due to the unavailability of a fast and trustworthy way to retrieve information on the product's provenance. Considering all the above, together with the rapid technological development adopted in value chain areas, we observe a significant increase in emerging innovations that lead the way for new digital traceability systems by taking advantage of information and communication technology (ICT), Radio-Frequency Identification (RFID) sensors, Internet of Things (IoT), blockchains and more. In this context, distributed ledger technologies (DLT) such as blockchain offer a solution to many existing problems, but simultaneously pose new challenges as well.  
Over the last few years, a rapid increase regarding the practical and effective application of DLT in the agri-supply chain has been under the spotlight in the academic and food industry communities [1]. In addition, the establishment of the Industry 4.0 paradigm renders DLT a promising area for further research in the future. In this paper, the authors identify, gather and present a thorough literature review of ongoing scientific papers, technical publications, research projects and pilot integrated and commercial platforms using blockchain for traceability in the agriculture supply chain. The main target of this paper is to conduct an analysis of the research activities performed over the last years and demonstrate a taxonomy of different ideas and a chronological presentation of useful studies. This will serve as the basis for conducting further research in order to better address the challenges that still exist in the domain.  
The remainder of the paper is organized as follows: Section 2 presents an overview of the definitions, levels, tools and advantages of traceability; Section 3 provides a brief review about the blockchain technology; Section 4 includes a literature review of related activities to date regarding the integration of blockchain into traceability systems; Section 5 presents relevant existing commercial applications; Section 6 highlights challenges and future prospects; Section 7 concludes the paper.