Demand of high-performance engineering thermoplastics in industrial countries drives the development of this class of materials in over 30 years. They display high heat distortion temperature, high modulus and strength, dimensional stability, chemical resistance, inherent flame retardancy and share the commonality of high glass transition. The observed variations of T-g with composition are compared with the postulated using Van Krevelen's structural group contribution theory. The predicted T-g does not differentiate the variations affected by moisture and crystallinity. The 'wet' T-g's are determined by modulated temperature-DSC (MT-DSC), while the 'dry' or the constained T-g due to orientation and/or annealing is detectable by standard DSC. The glass transitions of the constrained domains in highly crystalline polymers can be followed by DMA, DEA and thermal expansibility.