The Alf (also: "Alfbach") is a nearly 52 km long, orographically left tributary of the Moselle.
The Alf rises about 1 km northeast of Hörscheid in the Volcanic Eifel. From its source at 549 m above sea level, the Alf initially flows in a southerly direction. The first town on its route is Darscheid, past whose eastern edge it flows. The next towns directly along its course are Gillenfeld and Strohn. In its upper reaches up to Strohn, the Alf flows for a long time in a wide and broadly gravelled valley, which is followed from near Steiningen to just before Udler, which is also outside the valley basin, briefly by the A 48 motorway and then for a longer period by the A 1 motorway, and a little later, starting at the Mehren industrial estate and ending at Gillenfeld, the Maare-Mosel cycle path, which runs there on the route of the former railway line from Wittlich to Daun.
In the following middle section, which continues southwards to Bausendorf, the course of the Alf is quite winding and is more deeply embedded in the Eifel hills; there are only mills on the river.
While the direction of the river was predominantly south up to Bausendorf, it turns east there and flows through the towns of Kinderbeuern and Bengel, south of the Kondelwald. Around 3.5 km downstream from Bengel, it abruptly turns north, as the ridge with the Reiler Hals blocks the further route eastwards to the Moselle, which is only 500 m away. After the Alf has taken in the Üßbach from the northwest, by far its largest tributary, whose partial catchment area even exceeds its own up to that point, it breaks its way eastwards for less than three kilometres through the Moselle mountains and then flows into the Moselle from the left in Alf in the Cochem-Zell district at 92 m above sea level.
After a 52 km long course with an average bed gradient of 8.8 ‰, the Alf flows into the river 459 meters below its source.