Condition monitoring of gearboxes has been increasingly demanded in machinery maintenance. Many research efforts have been devoted to developing efficient and effective techniques for gear spalling detection. Most of them rely on predefined spalling and results are not representative in particular cases. This thesis presents a progressive development of the techniques to detect an irregular spall in a gearbox. Results show that well-adapted techniques such as amplitude demodulation and wavelet map are prominent in describing the amplitude modulation caused by a spall. Phase demodulation indicates the phase variation introduced by a spall. A fault indicator based on the ratio of spectral power is proposed to monitor the development of the spalling. An enhanced empirical mode decomposition (EMD) and Teager's Kaiser Energy Operator (TKEO) is developed to effectively extract the critical characteristics of spalling diagnosis. The relation between the digital signatures and the physical state of gear is comprehensively discussed.