This summer school is the sixth "Encuentro Colombiano de Combinatoria (ECCO)” (Colombian Meeting on Combinatorics). Previous ones took place in Bogotá, Colombia in 2003, 2008, 2012, and 2014, and in Medellín, Colombia in 2016. The main objectives of the school are to bring young mathematicians from Latin America, Canada, USA and Europe into close contact with each other along with world experts in various fields of combinatorics and related areas and to promote mathematics among young students in a motivating environment.