This paper presents the main circuit design considerations for power-efficient and safe implantable electrical neurostimulators. Related to medical applications, low-frequency (LF) stimulation for generating new action potentials and kilohertz-frequency alternating current (KHFAC) for blocking unwanted neural activity are introduced, respectively. For implantable medical devices, the choice of energy source type is important as it has an influence on the total size of the device and device comfort, thereby affecting the quality of life of the patients. In order to lengthen the lifetime of the stimulator, power-efficient designs using the ultra-high frequency (UHF) pulsed technique are proposed. To avoid tissue damage and electrode degradation caused by residual charge on the electrode-tissue interface (ETI), charge balancing (CB) techniques are adopted. Active CB control is shown to be a promising method both for LF and KHFAC stimulation.