An analysis is given of the effect of additive and multiplicative noise in the data and servo channels of both write-once and magneto-optic recording systems. Electronic (additive) noise is most severe in the magneto-optic data channel, and laser noise is of particular concern in both the sum tracking channel and the write-once data channel. In a typical optical memory system, the maximum laser light fluctuation should never exceed 10% of the average light level. In the differential channel of such a system, the laser noise should be further reduced by another order of magnitude. In the data channel, nonreturn-to-zero (NRZ) encoding has the greatest immunity to noise. Analysis is made of the extent to which the S/N margin is lost when various run-length-limited codes are used, for both additive and multiplicative noise.