Depending on how it is classified the Hydrangea family has 8 to 18 genera with 115 to over 250 species.
Philadelphaceae is seen as a synonym (Mabberley) or as a separate family.
Deutzia, Dichroa, Hydrangea and Philadelphus are well known as garden plants and/or cut flowers and
    there are many hybrids and cultivars of them.

Hydrangeaceae are mostly shrubs with some subshrubs, herb, lianas (woody vines) and trees.
They can be prostrate, spreading or erect and evergreen or deciduous.
The mostly erect stems are under 3 m long and the bark peels off them in strips.
Small twigs are smooth or have simple or branched hairs.

The mostly opposite leaves may or may not have petioles.
The petiole bases may be joined around the stem.
Some genera have alternate leaves but they are rarely whorled.
The blades can be ovate, obovate, elliptic or lanceolate and some are palmately lobed.
The edge can be smooth, shallowly lobed (crenate) or have teeth.
Simple hairs may be present and they may have small tubercles or longer extensions.
Venation is mostly pinnate and cross veins are present.

Terminal or axillary inflorescences can be a single flower or a variously arranged, branched or
    unbranched cluster or sometimes a head.
An inflorescence can have hundreds of flowers.

The small bisexual flowers at the centre of a cluster are radially symmetric.
The larger sterile ones at the edge are bilaterally symmetric with petal-like sepals.
Flower parts are typically in 4’s or 5’s and rarely up to 10 or 12.

There may be no hypanthium but when present it can be free or fused to the ovary to various degrees.
There is almost always a nectary above the ovary.

The sepals are typically free but in some species their bases are fused.
In some flowers the sepals are enlarged and petal-like.
The petals are also mostly free of each other.

The 4 or 8 (up to 200) fertile stamens are usually free of each other but the filament bases may be fused.
The filaments may have wide, flat bases and they sometimes have 2 appendages at the top.
The dorsi or basi-fixed anthers open inwards via longitudinal slits.

The inferior or partly inferior ovary is formed from 2 to 5 (12) fused carpels.
There may be 1 locule or partition walls can divide it into up to 5 (12) locules.
Each locule can have up to 50 or 100 ovules with axile or parietal placentation depending on the
    presence or absence of partitions in the ovary.
There are 1 to 5 (12) styles that are free or partly joined.

Fruit are mostly a dehiscent capsule or a berry.
Capsules can be loculicidal, septicidal or open via a pore at the top of the ovary between the
    bases of the styles which may or may not persist.
The small seeds may have a wing.