Alonzo Church was undeniably one ofthe intellectual giants of theTwenti- eth Century . These articles are dedicated to his memory and illustrate the tremendous importance his ideas have had in logic, mathematics, comput er science and philosophy . Discussions of some of thesevarious contributions have appeared in The Bulletin of Symbolic Logic, and th e interested reader is invited to seek details there . Here we justtry to give somegener al sense of the scope, depth, and value of his work. Church is perhaps best known for the theorem, appropriately called " C h u r c h's Theorem," that there is no decision procedure forthelogical valid- ity of formulas first-order of logic . A d ecision proce dure forthat part of logic would have come near to fulfilling Leibniz's dream of a calculus that could be mechanically used tosettle logical disputes . It was not to . be It could not be . What Church proved precisely is that there is no lambda-definable function that can i n every case providethe right answer, ' y e s ' or ' n o', tothe question of whether or not any arbitrarily given formula is valid .