This chapter presents a study of the impact of conditioning period on structural analysis results obtained from ground motions selected using the Conditional Spectrum concept. The Conditional Spectrum provides a quantitative means to model the distribution of response spectra associated with ground motions having a target spectral acceleration at asingle conditioning period. One previously unresolved issue with this approach is how to condition this target spectrum for cases where the structure of interest is sensitive to excitation at multiple periods due to nonlinearity and multi-mode effects. To investigate the impact of conditioning period, we perform seismic hazard analysis, ground motion selection, and nonlinear dynamic structural analysis to develop a "risk-based" assessment of a 20-story concrete frame building. We perform this assessment using varying conditioning periods and find that the resulting structural reliabilities are comparable regardless of the conditioning period used for seismic hazard analysis and ground motion selection. This is true as long as a Conditional Spectrum (which carefully captures trends in means and variability of spectra) is used as the ground motion target, and as long as the analysis goal is a risk-based assessment that provides the annual rate of exceeding some structural limit state (as opposed to computing response conditioned on a specified ground motion intensity level). Theoretical arguments are provided to support these findings, and implications for performance-based earthquake engineering are discussed.