Organic electrochemical transistor (OECT) has high transconductance, low operating voltage, and good biocompatibility. It has emerged as a promising technology for chemo/biosensors, bioelectronics, neuromorphic computing, etc. Compared with the abundant high-performance p-type OECT materials, n-type and ambipolar OECT materials are rare and more importantly, their performances lag far behind, which largely limited the development of OECT-based logic circuits. The past few years have witnessed the fast development of n-type and ambipolar OECT materials and their applications, thanks to the efforts of novel material design and device optimization. In this review, we aim to provide an introduction to the working principles of n-type and ambipolar OECTs and survey their recent developments and applications. We will also discuss the challenges and issues that need to be addressed for the future advancement of the field.