Wyner's wiretap channel is generalized to the case when the sender, the receiver and the eavesdropper have multiple antennas. We consider two cases: the deterministic case and the fading case. In the deterministic case, the channel matrices of the intended receiver and the eavesdropper are fixed and known to all the nodes. In the fading case, the channel matrices experience block fading and the sender has only the intended receiver's channel state information (CSI) and statistical knowledge of the eavesdropper's channel. For the deterministic case, a scheme based on the generalized-singular-value-decomposition (GSVD) of the channel matrices is proposed and shown to achieve the secrecy capacity in the high signal-to-noise-ratio (SNR) limit. When the intended receiver has only one antenna (MISO case) the secrecy-capacity is characterized for any SNR. Next, a suboptimal "artificial noise" based scheme is considered. Its performance is characterized and observed to be nearly optimal in the high SNR regime for the MISO case. This scheme extends naturally to the fading case and results are reported for the MISO case. For the independent Rayleigh fading distribution as we simultaneously increase the number of antennas at the sender and the eavesdropper, the secrecy capacity approaches zero if and only if the ratio of the number of eavesdropper antennas to transmitter antennas is at least two.