As compared to constant-stress accelerated life-tests (CSALTs), step-stress accelerated life-tests (SSALTs) are known to be more efficient and would require fewer test units. This chapter provides a detailed discussion on the optimal designs of SSALTs for one-shot devices with exponential and Weibull lifetime distributions. It aims to determine the optimal setting of design parameters, such as inspection times and sample allocation, by minimizing some suitable criterion, such as the asymptotic variance of the maximum likelihood estimator of a parameter of interest ϕ. The chapter presents the optimal design of simple SSALTs for electro-explosive devices. It also presents an application of the optimal design for grease-based magnetorheological fluids (G-MRFs). A sample of 25 G-MRFs is sufficient for the reliability estimation.