The invention relates to a filter device for filtering a hydraulic fluid comprising a housing having fluid through-openings for hydraulic fluid to be filtered to flow into the housing and for filtered hydraulic fluid to flow out of the housing, and having an assembly opening for inserting a filter element into the housing, the assembly opening being closable by a cover. In order to further develop the filter device such that it can be selectively configured in a cost-effective manner as a suction filter, return filter or return-suction filter, it is proposed in accordance with the invention that different filter elements be insertable into the housing, and that different functional modules be insertable into the housing or connectable to the housing.