libiec61850 is an open-source (GPLv3) implementation of an IEC 61850 client and server library implementing the protocols MMS, GOOSE and SV. It is implemented in C (according to the C99 standard) to provide maximum portability. It can be used to implement IEC 61850 compliant client and server applications on embedded systems and PCs running FreeBSD, Linux, Windows, and MacOS. Included is a set of simple example applications that can be used as a starting point to implement own IEC 61850 compliant devices or to communicate with IEC 61850 devices. The library has been successfully used in many commercial software products and devices.