Electro-conjugate fluid (ECF) is a functional fluid that belongs to the category of smart materials. ECFs can convert electrical energy into mechanical energy without relying on mechanical moving parts. This unique capability has positioned ECFs at the forefront of developing next-generation technologies such as microfluidics, soft robotics, and energy-efficient active systems. This chapter provides a comprehensive discussion on key aspects of ECF, including fundamental concepts and characteristics observed under high electric fields. Furthermore, ECF applications across various fields are discussed while also highlighting certain limitations in their engineering application.