The existence of binary sequences with specific aperiodic autocorrelation and cross correlation properties is investigated. Relationships are determined among the size of a sequence set, the length of the sequences n, the maximum autocorrelation sidelobe magnitude

, and the maximum cross correlation magnitude

. The principal result is the proof of the existence of sequence sets characterized by certain combinations of

, and

. The proof makes use of a new lower bound to the expected size of sequence sets constructed according to an explicit "random coding" procedure. For large

, the sequence set size is controlled primarily by the cross correlation constraint

. Two consequences of the existence theorem are 1) a demonstration that large sequence sets exist for which the maximum autocorrelation sidelobe and cross correlation magnitudes vanish almost as fast as the inverse square root of the sequence length

a new proof of the Gilbert bound of coding theory.