Alonzo Church was an influential American mathematician and logician, best known for his work in the foundations of computer science. He developed the concept of the lambda calculus, a formal system that helps in understanding functions and computation. His ideas laid the groundwork for modern programming languages and theoretical computer science. In addition to his work on the lambda calculus, Church contributed to the development of Church's Thesis, which proposes that any computation can be performed by a Turing machine. This idea connects his work to that of Alan Turing, another pioneer in computer science, and has had a lasting impact on how we understand algorithms and computation today.