Tensile testing of solution annealed and thermally aged (755 K for various durations in the range of 0.25-100 h) specimens of M250 grade maraging steel has been carried out along with acoustic emission (AE) monitoring. Results have shown that strength increases and ductility decreases upon ageing up to 10 h and this has been attributed primarily to the precipitation of Ni3Ti. Continued increase in strength up to 40 h of ageing has been attributed primarily to the precipitation of Fe2Mo in addition to Ni3Ti. Increase in ductility for 10-40 h of ageing has been attributed to dissolution of needle like Ni3Ti precipitates and formation of fine spherical Fe2Mo. Ageing beyond 40 h decreases strength and increases ductility due to the reversion of martensite to austenite and coarsening of the precipitates. The AE generated during tensile deformation depends on the ageing time. Increased occurrence of shearing of the precipitates by dislocations and increased brittleness of the matrix up to 10 h ageing increases the AE. The decrease in the AE beyond 10 h of ageing is due to the occurrence of deformation by Orowan looping, dissolution of Ni3Ti precipitates and austenite reversion. The scanning electron microscopy (SEM) of the fracture surfaces has shown ductile fracture characterized by dimples and changes in the size and shape of the dimples with ageing time.