We argue that the European Union Emissions Trading System (EU ETS) has evolved into a hybrid of two design variants, allowance trading (cap-andtrade) and credit trading (performance standard rate trading), with an added feature of industry support to minimize carbon leakage. In particular the current rules tying free allowances to production capacity expansion, plant closure and capacity use have transformed the efficient cap-and-trade program that stood at the origins of the EU ETS into a system that even surpasses credit trading in paying hidden product subsidies to firms. This combination of rules encourages an inefficiently high level of investment in production capacity and an inefficiently high output in industries exposed to international competition. The result is a sub-optimal EU Emissions Trading 'Hybrid' (which we therefore label as 'EU ETH').European Parliament and the Council of the European Union have since adopted an amending Emissions Trading Directive introducing some new rules to apply in the current and third period 2013-2020 (Directive 2009/29/EC). Early 2018 the European Council approved another reform of the EU Emissions Trading System (EU ETS) for the next and fourth period 2021-2030 (Directive (EU) 2018/410). In this paper we aim to examine which rules have changed between the first and the third period and how efficiently the reduction of CO 2 emissions is currently being achieved under the EU ETS. Our economic analysis thus focuses on legal amendments made to the EU ETS so far (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020), leaving an analysis of the next period's rules (2021-2030) for future research.We start with a survey of the economic-analytical literature on emissions trading and use that knowledge to assess the efficiency of the original EU Emissions Trading Directive of 2003 and its deterioration due to the changes after 2013. We will explain why the additional rules applying to capacity expansion and plant closure lead to sub-optimal outcomes. A next set of new rules added an extra criterion for the allocation of free allowances by tying their number to the level of operations relative to capacity. Although this was possibly meant as a reparation, we will argue that this has in fact done further damage to the efficiency of the scheme. The various rather complex criteria for the allocation of free allowances did already receive attention by a number of scholars, such as Ahman and Holmgren (2006), Ellerman (2008), Christin et al. (2011), Meunier et al. (2014) and Branger et al. (2015. However, the niche of our paper is that we: (a) perform the analysis based on the conceptual distinction between allowance trading (cap-and-trade) and credit trading (performance standard rate trading); (b) use the current laws and regulations to model firm behaviour under the amended EU ETS; and (c) clarify -in two tables -how the successive revisions of rules in the past have affected corporate decisions and hence the efficiency of EU ETS.In Section...