Droplets on a liquid substrate (‘liquid lenses’) play an important role in various branches of engineering, including microfluidics, chemical engineering, environment protection, etc. In the present paper, we discuss basic phenomena characteristic for liquid lenses. We recall classical results on the shape of an equilibrium droplet and the kinds of droplet wetting. We overview briefly the main theoretical approaches used for the analysis of droplet dynamics, discuss the phenomena accompanying a droplet impact, physical effects used for droplet manipulations, and the factors that determine the interaction between droplets. We describe the main types of droplet instabilities leading to oscillations, self-propulsion, and disintegration of droplets. Some promising directions of further research are listed.

液体基质上的液滴（“液体透镜”）在工程的各个分支中都起着重要作用，包括微流体，化学工程，环境保护等。在本文中，我们讨论了液体透镜的基本现象。我们回想起关于平衡液滴形状和液滴润湿类型的经典结果。我们简要概述了用于液滴动力学分析的主要理论方法，讨论了伴随液滴撞击的现象，用于液滴操纵的物理效应以及确定液滴之间相互作用的因素。我们描述了导致液滴振荡，自推进和液滴崩解的液滴不稳定性的主要类型。列出了一些有希望的进一步研究方向。