Speech coding at 64 and 32 Kb/s is well developed and standardized. The next bit rate of interest is at 16 Kb/s. Although. standardization has yet to be made, speech coding at 16 Kb/s is fairly well developed. The existing coders can produce good quality speech at rates as low as about 9.6 Kb/s. At present the major research area is at 8 to 4.8 Kb/s. This work deals first of all with enhancing the quality andkcomplexity of some of the most promising coders at 16 to 9.6 Kb/s as well as proposing new alternative coders. For this purpose coders operating at 16 Kb/s and 12 to 9.6 Kb/s have been grouped together and optimized for their corresponding bit rates. The second part of the work deals with the possibilities of coding the speech signals at lower rates than 9.6 Kb/s. Therefore, coders which produce good quality speech at bit rates 8 to 4.8 Kb/s have been designed and simulated. As well as designing coders to operate at rates below 32 Kb/s. it is very important to test them. Coders operating at 32 Kb/s and above contain only quantization noise and usually have large signal to noise ratios (SNR). For this reason their SNR's may be used for comparison of the coders. However, for the coders operating at 16 Kb/s and below this is not so and hence subjective testing is necessary for true comparison of the coders. The final part of this work deals with the subjective testing of 6 coders, three at 16 Kb/s and the other three at 9.6 Kb/s.