This paper examines the decentralization of Ethereum's builder market, a critical component of the blockchain that handles block production. Despite being designed to be decentralized, the builder market has become highly centralized, with three builders controlling over 90% of blocks. The authors investigate the reasons behind this centralization and the implications for MEV-Boost auctions, which are used to mitigate the negative impacts of Maximal Extractable Value (MEV). The study uses a large-scale dataset of MEV-Boost auctions, collected from September 2022 to August 2023, to analyze the openness, competitiveness, and efficiency of the builder market. Key findings include: 1. **Market Entry Barriers**: New builders face significant entry barriers, primarily due to access to private order flows, which contribute over 60% of block value in more than 50% of blocks. Private order flow providers impose reputation requirements, such as market share, which are challenging for new builders to meet. The cost of subsidizing to establish a 1% market share can be up to 1.4 ETH, a substantial amount for small builders. 2. **Inequality in Block-Building Capabilities**: There is significant inequality in builders' abilities to extract MEV, with top builders having a much higher capacity than middle and tail builders. This inequality worsens as MEV increases. 3. **Competitiveness and Efficiency**: While 88.84% of auctions were competitive, uncompetitive auctions resulted in total losses of 221.09 ETH for proposers, representing 0.98% of their total gains. Only 79.74% of auctions were efficient, with over half of the inefficient auctions caused by block subsidization. 4. **Future Directions**: The paper suggests that future developments should aim to reduce or eliminate trust requirements in MEV-Boost auctions, such as through community supervision or Trusted Execution Environments (TEEs), to enhance decentralization. The paper provides a comprehensive analysis of the builder market in Ethereum, highlighting the challenges of centralization and the need for improved mechanisms to ensure fairness and efficiency. The findings offer insights into the dynamics of MEV-Boost auctions and suggest potential solutions to address the issues of centralization and trust.This paper examines the decentralization of Ethereum's builder market, a critical component of the blockchain that handles block production. Despite being designed to be decentralized, the builder market has become highly centralized, with three builders controlling over 90% of blocks. The authors investigate the reasons behind this centralization and the implications for MEV-Boost auctions, which are used to mitigate the negative impacts of Maximal Extractable Value (MEV). The study uses a large-scale dataset of MEV-Boost auctions, collected from September 2022 to August 2023, to analyze the openness, competitiveness, and efficiency of the builder market. Key findings include: 1. **Market Entry Barriers**: New builders face significant entry barriers, primarily due to access to private order flows, which contribute over 60% of block value in more than 50% of blocks. Private order flow providers impose reputation requirements, such as market share, which are challenging for new builders to meet. The cost of subsidizing to establish a 1% market share can be up to 1.4 ETH, a substantial amount for small builders. 2. **Inequality in Block-Building Capabilities**: There is significant inequality in builders' abilities to extract MEV, with top builders having a much higher capacity than middle and tail builders. This inequality worsens as MEV increases. 3. **Competitiveness and Efficiency**: While 88.84% of auctions were competitive, uncompetitive auctions resulted in total losses of 221.09 ETH for proposers, representing 0.98% of their total gains. Only 79.74% of auctions were efficient, with over half of the inefficient auctions caused by block subsidization. 4. **Future Directions**: The paper suggests that future developments should aim to reduce or eliminate trust requirements in MEV-Boost auctions, such as through community supervision or Trusted Execution Environments (TEEs), to enhance decentralization. The paper provides a comprehensive analysis of the builder market in Ethereum, highlighting the challenges of centralization and the need for improved mechanisms to ensure fairness and efficiency. The findings offer insights into the dynamics of MEV-Boost auctions and suggest potential solutions to address the issues of centralization and trust.