Abstract: The invention relates to a modular behind-the-ear hearing aid (BTE) with a variably small size. The BTE can be designed for different performance levels and a different range of functions for manufacturing and maintaining the BTE easily and cheaply. The modular behind-the-ear hearing aid can be assembled from two units, wherein the first unit is a base unit which provides the behind-the-ear hearing aid with a basic range of functions of a particular series of the behind-the-ear hearing aid, and the second unit is a plug-in unit which can be plugged into the base unit and which provides the behind-the-ear hearing aid with an additional range of functions in accordance with customers' wishes, wherein the basic range of functions of the base unit only forms an operational behind-the-ear hearing aid in conjunction with the additional range of functions of the plug-in unit.