One of the most effective solutions to integrate renewable energy into electrical power systems is the microgrid approach, where renewable sources as well as energy storages are usually interfaced with the ac bus of a microgrid through power electronics inverters. Meanwhile, in order to improve the power supply reliability, microgrids should be able to operate in islanded mode and switched off from utility grid when utility fails. Under this circumstance, the voltage quality of ac bus and the power sharing among these paralleled inverters are very crucial for the safe operation of system, which must and can only be handled by the coordinative control of the inverters. Communication-based coordinative control is widely used for paralleled inverters in conventional UPS systems. However, paralleled inverters in a microgrid are very often located at different sites, and communication-less coordinative control is much more attractive and competitive due to its higher reliability, less cost, and its plug-and-play nature for installation and maintenance. There are two types of communication-less coordinative control, i.e. master-slave control and droop control. The focus of this tutorial will be the droop control, which is more widely used because of its advantages over the master-slave control. The basic operation principles of droop control will be introduced with DC bus power grids as examples, through detailed illustrations based on the simplest system structure of 2 paralleled source converters and one common load. These principles will then be extended to AC bus power grids, where droop control is implemented in two channels: active power channel and reactive power channel. Several major technical issues that need to be dealt with in droop control will then be identified and some of them will be discussed extensively. An in-depth review of existing and recent work of droop control for paralleled inverters in islanded microgrids is then delivered. Topics that will be covered include power coupling introduced by distribution cables, reactive power sharing control, unbalanced and harmonics power sharing control, secondary control for frequency restoration, and comparison between droop control and Virtual Synchronous Generator (VSG) control. The targeting audience of the seminar would be engineers, graduate students and academia faculties who are interested in the topic. The level of the intended audience will be intermediate.