System identification of a scale structure in the laboratory preceded full scale studies of the Four Seasons office building (Sherman Oaks, CA) under forced vibrations and the Louis Factor building (UCLA) under earthquake and ambient vibrations. An advanced system identification algorithm, N4SID (numerical subspace state space system identification), is employed. The spurious modes generated due to sensor noise and measured errors are distinguished from the physical ones and disregarded through the use of ’¡Èstability diagrams’¡É. A finite element computer program is used to analyze each structure studied and analytical results are compared to experimental results.