Apparatus and methods for focusing transcranial ultrasound. The systems described herein are advantageous for noninvasive neuromodulation and other transcranial ultrasound applications such as high intensity focused ultrasound (HIFU). In particular, described herein are compound acoustic lens apparatus having a short focal length for use with a transcranial ultrasound system, systems including methods of using them. These compound lens assemblies allow transcranial stimulation of even superficial cortical regions of the brain for ultrasound neuromodulation with a compact, single transducer element system at low (e.g., 0.2 to 1 MHz) frequencies with relatively large diameter (e.g., >；15 mm) transducers applying 1 to 10 watts/cm2 of acoustic energy (spatial-peak, temporal-average intensity at the target brain region), and short focal length (e.g., between 15 and 35 mm).
