 We determine what information about failures is necessary and sufficient to 	solve Consensus in asynchronous distributed systems subject to crash 	failures. In <a href="http://www.cs.cornell.edu/Info/People/chandra/UnreliableFD.html">[CT91]</a>, we proved that <i>W</i>, a failure detector that 	provides surprisingly little information about which processes have 	crashed, is sufficient to solve Consensus in asynchronous systems with a 	majority of correct processes. In this paper, we prove that to solve Consensus, any failure detector has to 	provide at least as much information as <i>W</i>. Thus, <i>W</i> is indeed the weakest failure detector for solving Consensus 	in asynchronous systems with a majority of correct processes. <hr> Research supported by an IBM graduate fellowship, 	NSF grants CCR-8901780 and CCR-9102231, DARPA/NASA Ames 	grant NAG-2-593, grants from the IBM Endicott Programming Laboratory 	and Siemens Corp, and a grant from the Natural Sciences and 	Engineering Research Council of Canada. <hr> <i>Maintained by <a href="http://www.cs.cornell.edu/Info/People/chandra/home.html">tushar@watson.ibm.com</a></i>
