Smart contracts, originally introduced in the 1990s by Nick Szabo, have gained prominence with the rise of blockchain technology, and with the latest developments in algorithmic contracting. Their impact on contract law is evident, and therefore this paper analyzes from the English contract law perspective their formation phase in order to become legally enforceable. In addition, the analysis is carried out separately with respect to smart contracts whose formation phase is prior to their translation into code, from those in which algorithms are involved in the contract formation phase, which presents greater complexities. The purpose of this paper is to highlight the challenges presented by the legal framework of smart contracts in order to prepare the basis for future research on ways to address these concerns.