An Application-Specific Intergrated Circuit, or ASIC, is a type of computing hardware chip optimized for a particular use. ASICs are used in many Proof of Work blockchain networks, particularly Bitcoin. Since ASICs are dramatically more efficient in solving Proof of Work puzzles than are consumer hardware, the vast majority of Bitcoin mining power is from ASICs. The use of ASICs is controversial in the industry; projects such as Monero and Grin aim to minimize the use of ASICs, citing concerns over centralization of operating hash power and miner production (ie, Bitmain's market share in 2016-2019) as network security risks. However, ASICs increase the cost of 51% attacks and arguably align long-term incentives of large-scale mining operations with the protocol. This article provides an overview of the history and outlook for ASICs.