In this chapter, we evaluate the performance of STARSs under independent and coupled phase-shift models. We first introduce the most common performance metrics in the context of RIS-aided wireless communications. Then, for independent phase-shift STARSs, we analyze its diversity gain and compare it with conventional RISs. For coupled phase-shift STARSs, we propose three practical phase-shift configuration (PSC) strategies, namely, the primary-secondary PSC (PS-PSC), diversity-preserving PSC (DP-PSC), and T/R-group PSC (TR-PSC) strategies. Then, we derive the outage probability and power scaling law for each of the proposed PSC strategies. We also compare these strategies with the case of independent phase-shift STARS and STARS with random PSC. We further deduce the diversity orders of the users for all proposed strategies and show that, with the DP-PSC strategy, full diversity order can be simultaneously achieved for the users on both sides of the STARS.