Some examples include using an inventory tracking device to track the addition of inventory items to an inventory holder of an inventory system or removal of inventory items from the inventory holder. The inventory holder may include one or more trays to store one or more inventory items, with each tray including one or more bins that may be defined by dividers on the tray. The inventory tracking device may use inventory holder configuration data that identifies locations of the bins to determine that the inventory item was added to or removed from a particular bin of the inventory holder.