The ptychographic iterative engine (PIE), a recently developed lens-less imaging technique, provides a reliable access and solution to the phase problem. It has superior convergence speed and accuracy compared with conventional coherent diffraction imaging technologies. PIE is used extensively in various imaging and measurement fields owing to its unlimited field-of-view, high resolution, and high robustness to noise. In this review, we discuss the background and principle of PIE and summarize the major technological advances. The primary milestones of PIE in X-ray, electron, and visible light over the past decade are also discussed. Furthermore, we also elaborate on the latest PIE-based techniques and potential future developments and challenges.