The steady growth of the e-commerce industry, especially fuled by the pandemic, puts increased pressure on various warehouse operations. One potential approach to increase order picking efficiency is to reduce picker walking distances. The unique feature in mixed shelves storage warehouses is that items to be picked can be located in multiple storage positions in the warehouse. This results in a multi-layered optimization problem: Suitable positions must be selected as well as the shortest route between them has to be found. The goal of this thesis is to develop a heuristic solution for the picker routing problem in mixed shelves storage warehouses, to implement it and to test it against existing methods.