The Structural Time Domain Identification (STDI) toolbox for use with MATLAB(TM) is developed at Aalborg University, Denmark, based on the system identification research performed during recent years. By now, a reliable set of functions offers a wide spectrum of services for all the important steps of multivariate time domain system identification of time-variant as well as time-invariant civil engineering structures from ambient testing data. A graphical user interface (GUI) is also developed to make the toolbox more user friendly.