The Fils has its source on the Swabian Alb about two kilometers southwest of the town Wiesensteig. Its karst spring, the Filsursprung, lies at 624.9 m in the Hasental valley, which has remained close to nature. There are two more springs a little further down, the Kleiner Filsursprung on the right and the Hasenquelle on the left at the foot of the slope. The young river - initially running in a north-easterly direction - crosses the community Mühlhausen im Täle after Wiesensteig, where the Hollbach flows on the left side and the Bundesautobahn 8 crosses the valley. Then follows Gosbach with the mouth of the Gos on the right side. In Bad Ditzenbach the Hartel flows into the river from the left and the Ditz from the right. In Deggingen the longer Schinderbach flows into the Fils from the left and the short waters Sennenbach and Schüttebach from the right. At Reichenbach im Täle in its lowest valley the Fischbach and in Hausen an der Fils the Rohrbach flows into the Fils. In Bad Überkingen the Rötelbach coming from the right lets the Fils grow further. Through Geislingen an der Steige, the Filstal describes a 90 degree left turn, where the Eyb, which flows in from the northeast, joins in. The Eyb is the first longer and on the surface also more catchment area-rich tributary from the right side, after the streams running in from the left from the crumbling inner side of the Alb Eaves have so far regularly exceeded their counterparts on the edge of the flat Alb plateau in both characteristics. The Eyb river absorbs a northwestern tributary from Amstetten before its mouth, which is why Geislingen lies in a wide valley spider. From now on, the Fils also runs in a northwesterly direction through its outlet bay from the Alb into the Swabian Keuper-Lias Plains, where Kuchen and then at the tributary of the Marrbach Gingen lie. In the meantime, before the Alb, the right Lauter, the largest tributary of the Fils at all, flows into Süßen on its last west-southwestern course. From here on the Fils runs approximately westward in a constantly increasing distance from the step edge of the Alb to its left, which is why from now on the left and right tributaries become more similar in length and catchment area. In the now wide valley follow Salach, Eislingen with the Krumm from the right, the second largest tributary of the Fils, and the left Weilerbach, then Göppingen, Faurndau with the Marbach., Uhingen with the Blaubach, the Butzbach and the Nassach. The third last village at the river is Ebersbach an der Fils, where the name-giving Ebersbach flows in from the right as well as the following Kirnbach shortly before the community and district border to Reichenbach an der Fils. In this place Reichenbach and Lützelbach flow in from the right and finally the Talbach from the opposite side. Thereupon the Fils reaches Plochingen, where it flows at 248.3 m after about 63 kilometres and almost 377 metres of altitude below its source from the right into the left curve of the Neckar there; it thus has a mean bottom gradient of about 6°. ‰. Its catchment area covers about 707 km². It is the fifth largest Neckar tributary by length and the fourth largest by catchment area.
