The bilinear maximal operator defined below maps $L^p\times L^q$ into $L^r$ provided $10}\frac1{2t}\int_{-t}^t\abs{f(x+y)g(x-y)} dy.$$ In particular $Mfg$ is integrable\thinspace if $f$ and $g$ are square integrable, answering a conjecture posed by Alberto Calder\'on.
