According to the "What does a geocache look like?" section of Groundspeak's Geocaching Guide, a microcache is a cache whose container is less than 100ml (approximately 3oz). Most are too small to hold a pen or pencil, and many are too small to hold anything but a log sheet. If there are trade items in such caches, they tend to be very small items, such as coins, signature tokens, and charms.
Microcaches come in many differing types of containers. Film canisters are common, as are magnetic key safes. Exceptionally small micros are known as nano caches.
Ideally, microcaches should be hidden in locations where larger containers could not be hidden easily, such as urban parks and plazas. Microcaches in urban settings are often designed and camouflaged for a specific location. Such caches are often called urban micros.
One of the elements that microcaches bring to geocaching is the extra challenge of locating it, as often the vicinity of the cache does not indicate the location of the cache. In many cases, the element of stealth is required as the cache may be hidden in a highly-trafficked spot, which adds to the challenge of locating the cache while non-cachers are present.
Some geocachers dislike microcaches because they are too small to hold interesting trade items. They especially dislike microcaches placed at locations that could support larger cache containers. Others complain that microcaches can be hidden with very little preparation, allowing a geocacher to hide an inordinate number of poorly thought-out caches (e.g., lamp post caches), as opposed to those who take extra effort to prepare larger caches.
Others point out that many microcaches are designed with detail and significant planning, and that microcaches are usually hidden where larger caches would be inappropriate. Some argue that microcaches allow geocachers to have the extra challenge of microcache hunting versus hunting for regular-sized caches.