In this article, diffusion of moisture in adhesively bonded composite joints is discussed and analysed experimentally, analytically and numerically. The experimental studies concentrate on moisture diffusion in adhesive films and in unidirectional and multidirectional composite substrates exposed to two different conditioning environments, namely 45degreesC/85% RH and 90degreesC/97% RH for the absorption studies and 90degreesC/ambient for the desorption studies. The coefficients of diffusion are determined from the water uptake plots. The analytical solutions for diffusion in joints with impermeable adherends are based on the classical theory of diffusion and are used to derive equations in two-dimensions for different adhesive fillet shapes, namely radiused fillet, triangular fillet and rectangular fillet. In the finite element analysis, the diffusion of moisture from the composite substrates into lap-strap joints is also taken into account. Both unidirectional and multidirectional composites are considered, as well as two different fillet shapes, i.e., rectangular and triangular fillet. A comparison between the results obtained using FEA and those obtained using the analytical solution is made. Finally, fatigue test data for lap-strap joints aged and tested in different environments is presented and a tentative link between fatigue threshold and water concentration at the site of failure initiation is made, indicating a semi-empirical method of predicting the strength of joints subjected to moisture-induced degradation.