Modified frequency modulation (MFM) is a run-length limited (RLL) line code used to encode data on most floppy disks and some hard disk drives. It was first introduced on hard disks in 1970 with the IBM 3330 and then in floppy disk drives beginning with the IBM 53FD in 1976.

MFM is a modification to the original frequency modulation encoding (FM) code specifically for use with magnetic storage. MFM allowed devices to double the speed data was written to the media as the code guaranteed only one polarity change per encoded data bit. For this reason, MFM disks are typically known as "double density", while the earlier FM became known as "single density".

MFM is used with a data rate of 250–500 kbit/s (500–1000 kbit/s encoded) on industry-standard 5+1⁄4-inch and 3+1⁄2-inch ordinary and high-density floppy diskettes. MFM was also used in early hard disk designs, before the advent of more efficient types of RLL codes. Outside of niche applications, MFM encoding is obsolete in magnetic recording.

Magnetic storage devices, like hard drives and magnetic tape, store data not as absolute values, but in the changes in polarity. This is because a changing magnetic field will induce an electrical current in a nearby wire, and vice versa. By sending a series of changing currents to the read/write head while the media moves past it, the result will be a pattern of magnetic polarities on the media that change where the data was a "1". The exact nature of the media determines how many of these changes can occur within a given surface area, and when this is combined with the nominal speed of movement, it produces the maximum data rate for that system.

Disk drives are subject to a variety of mechanical and materials effects that cause the original pattern of data to "jitter" in time. MFM as a run-length limited code limits the distance between recorded transitions so the jitter does not cause a transition to be misaligned in time, thereby causing a data error. Other limitations defined by the media place additional constraints on the way the data is recorded. A diverse range of suitable encodings, known generally as line codes, have been developed for this purpose. Their suitability depends on the media or transmission mechanism being used.

Frequency modulation encoding (FM) was the first widely used system to perform this operation on disk drives. The drive controller includes an accurate clock running at half the selected data rate of the disk media. When data is written to the disk, the clock signal is interleaved with the data. On reading, the clock signals are used as short-term triggers to time the presence or lack of a following signal that represents the data bits.

The upside to the FM approach is that it is extremely easy to implement the writing circuitry, and the clock recovery on reading is also relatively simple. The downside is that it uses up half of the disk surface for the clock signal, thus halving the total amount of data the disk can store. This led to the development of new forms of encoding that were more efficient.

Modified frequency modulation encodes the clock signal and the data in a single "clock window". Unlike FM, a clock bit is only written when needed to achieve synchronization when both current and preceding data bits are not set. On average, MFM achieves double the information density of FM.