After the advent of compact disk the end user has started to expect high quality output from any digital audio application. The increasing demand of audio transmission and exchange over the internet and the growing trend of storage/playback of high quality audio on digital computer systems persistently call for developing new and improved audio coding techniques which can meet the contradicting demands of high quality and low bit rate both at the same time. Perceptual audio coding which employs the process of irrelevancy reduction through exploitation of human auditory perception has a central place in modern audio coding scenario. In this thesis several new techniques are developed within the paradigm of perceptual audio coding to enhance the coding efficiency and produce better output quality at reduced bit rates. The performance of developed techniques is evaluated by employing both subjective and objective measures recommended by the ITU-R. Moreover a coding scheme is developed which integrates the advantages of separately available sophisticated speech and audio coding techniques within a single algorithm. This codec is intended to cater for the needs of present multimedia scenario which contains mixed speech and audio signals thus requiring a single codec to handle all signal types for optimal coding efficiency.