A review on microstructured mixer devices and their mixing principles concerning miscible liquids (and gases) is given. This is supplemented by the description of typical mixing element designs, methods for mixing characterisation, and application fields. The mixing principles applied can be divided in two classes relying either on the pumping energy or provision of other external energy to achieve mixing, termed passive and active mixing, respectively. As far as passive mixing is concerned, devices and techniques such as Y- and T-type flow-, multi-laminating-, split-and-recombine-, chaotic-, jet colliding-, recirculation flow-mixers and others are discussed. Active mixing can be accomplished by time-pulsing flow owing to a periodical change of pumping energy or electrical fields, acoustic fluid shaking, ultrasound, electrowetting-based droplet shaking, microstirrers, and others.