Leadlights, leaded lights or leaded windows are decorative windows made of small sections of glass supported in lead cames. The technique of creating windows using glass and lead came to be known as came glasswork. The term 'leadlight' could be used to describe any window in which the glass is supported by lead, but traditionally, a distinction is made between stained glass windows and leadlights; the former is associated with the ornate coloured-glass windows of churches and similar buildings, while the latter is associated with the windows of vernacular architecture and defined by its simplicity.

Since the traditional technique of setting glass into lead cames is the same in both cases, the division between 'leadlights' and 'stained glass' became less distinct during the late 20th century. The terms are now often incorrectly used interchangeably for any window employing this technique, while the term 'stained glass' is often applied to any windows, sculptures or works of art using coloured glass.

Traditionally, leadlight windows differ from stained glass windows principally in being less complex in design and employing simpler techniques of manufacture. Stained glass windows, such as those commonly found in churches, usually include design components that have been painted onto the glass and fired in a kiln before assembly. The extra time and cost employed in painting and firing the glass usually prohibited its use in domestic architecture. While stained glass windows are found principally in churches and ornate buildings, leadlight windows, which rarely employ painted components, are much more common, and from the 1860s to the 1930s were a regular architectural feature in many private houses and cottages, where their style is often a clue to the age of the building.

Unlike stained glass windows which are traditionally pictorial or of elaborate design, traditional leadlight windows are generally non-pictorial, containing geometric designs and formalised plant motifs. Leadlight windows almost always employ the use of quarries, pieces of glass cut into regular geometric shapes, sometimes square, rectangular or circular but most frequently diamond-shaped, creating a "diaper" pattern.

A further difference between traditional stained glass and leadlight is that the former almost always has painted pictorial details over much of the glass, requiring separate firing after painting by the artist. In traditional leadlight this is not the case, painted quarries being separately produced and leaded in with those of plain glass, in the form of armorial crests and occasionally small scenes painted in grisaille (grey). Quarries painted in grisaille were employed both in the Medieval period and in the 19th century, the most famous ancient windows to have been decorated in this manner being in York Minster; these have inspired many 19th century imitations painted with little birds.

Quarries may be mould-cast into patterns such as fleur de lys and imprinted with black and yellow stain. Used extensively during the 19th century in England and Commonwealth countries, these quarries are often the product of a single studio, James Powell and Sons of Whitefriars. Another form of decorative quarry is the etched or engraved quarry, which is made of flashed glass, most often ruby red or royal blue over a transparent layer. It then has a design cut into it using either acid or a lathe, the character of the resultant design differing accordingly. Etched quarries of Venetian glass are often employed, sometimes in conjunction with panels of stained glass, particularly in Italy and Eastern Europe. Lathe-cut quarries with a simple star-burst pattern are very common in the late 19th century domestic architecture of many regions, both in leadlighting and in simpler wooden-framed glazing.

The colours employed in leadlight windows may range from delicate pastels to intense hues. The glass used may be textured or patterned or bevelled (as in the small panel from the 1920s illustrated above). However, since they are generally non-pictorial, and are primarily to illuminate the interior, with or without a decorative function, the glass is usually of pale hue, or transparent.

The work of the leadlighter was essentially to provide windows that excluded the weather, but admitted light into buildings. Leadlight has been in use for over a thousand years, having its origins in the Roman and Byzantine windows that were made of thin sheets of alabaster set in armatures of wood or wrought iron. With the employment of small pieces of glass as the translucent material (rather than alabaster), lead "cames" of H-section were used to hold the glass in place, with the iron armatures being retained as support for larger windows.