Shimano Pedaling Dynamics, also known as SPD system is a design of clipless bicycle pedals. The SPD pedal was released by Shimano

Shimano

Shimano, Inc. is a Japanese multinational manufacturer of cycling components, fishing tackle, and rowing equipment.In 2005, the company had net sales of US $1.4 billion. Bicycle components provided 75% of its sales income...

 in 1990. While not the first, its innovation was its small cleat

Cleat (shoe)

Cleats or studs are protrusions on the sole of a shoe, or on an external attachment to a shoe, that provide additional traction on a soft or slippery surface...

which fitted in a recess in the sole of a shoe designed for SPD use. The shoe could be also used for walking whereas previous systems (other than the Cyclebinding system) had a large protruding cleat. Clipless pedals use a system of cleat retention which is quite similar to that of a downhill ski binding. Generally, clipless pedals are seen to be safer than the older style of pedal/shoe integration using toeclips, toestraps and shoeplates due to ease of disengagement.

Shimano makes a number of different SPD cleats, and not all cleats are compatible with all pedals, especially between "road" and "mountain" SPD product lines. Among otherwise-compatible cleats, there is an important distinction between black cleats ("single release") and silver cleats ("multi release"). The former can only be released from the pedal by twisting the heel outwards, while the latter can also be released by a sharp pull upwards and are therefore easier to use for novices.

There is a small allen key adjustment for the strength of heel twist to release, so beginners can have it 'soft' and easy to get out, and then tighten it up as they progress.