Necessary and sufficient conditions for the matrix equation XAX = B and for the n system of matrix equations XAX = B, 2 CijXD;j = K,, i=l,2, . . . ,m, to be solvable are proven in this paper. If solvable, all solutions of the equation or system are determined. These results are used to obtain particular solutions of the general quadratic matrix equation XAX+BX + XC = D.