The article presents mathematical and computational research dealing with the problem of stochastic interface defects occurring in composite materials between their constituents. A mathematical model of the periodic composite with such defects is presented in detail, as well as probabilistic numerical methods enabling computational experiments which are shown in a further part of the text. The ber-reinforced and laminated composite has been tested in numerical tests as well as the superconducting coil cable-four-component composite to verify how the structural defects considered, according to the model introduced, in uence the static behavior of the composites analyzed. All the results obtained and discussed in the article are summarized in concluding remarks which show the directions of further model development, while numerous references enable the reader to study the problem further.The rst mathematical theories dealing with composite materials assumed that their mechanical properties depend on elastic characteristics of the constituents [13]. It was an effect of the mathematical model that components of the composite were assumed to be perfectly bonded. However, experimental practice in composite engineering shows that the overall behavior of multicomponent structures is determined mainly by the quality of the component materials bonding [27]. The structural defects occurring on the composite interfaces are caused by the technological problems resulting generally from thermal incompatibility of composite constituents and interfacial stress concentrations. On the other hand, the quality of composite constituents bonds is worsened by the different kinds of degradation processes caused by the water penetration of composite interfaces. The presence of water on the interfaces can cause swelling and plasti cation of the matrix and of the capillary tubes along the interface boundaries in the direction parallel to the ber orientation. The phenomena described above may lead to interphase debonding, interfacial sliding [22,33], and even to ber pull-out, and they have been described in detail in [20,32].All the interface phenomena have been worked out through numerous nite-element implementations based generally on the special interface-type nite elements [21,34,35],