A mirrorless interchangeable lens camera is a camera with an interchangeable lens that does not have a mirror reflex optical viewfinder. They are a subset of interchangeable lens cameras, a category that also includes single-lens reflex cameras (SLRs or DSLRs for digital models).

Mirrorless cameras are designed to have the advantage of smaller size, lighter weight, and lower cost than SLRs, while still allowing users to swap lenses, unlike most compact cameras.

Mirrorless cameras constituted five percent of total camera shipments in 2013. In 2015, mirrorless-cameras accounted for 26 percent of interchangeable-lens camera sales outside the Americas, but only about 16 percent in the U.S.

Mirrorless cameras are available with a variety of sensor sizes including 1/2.3", 1/1.7", 1", Micro 4/3, APS-C, and full frame. When sales of DSLRs fell in 2013, many camera manufacturers, including Nikon, responded by releasing many Mirrorless cameras including the lenses.

As of 2014, several mirrorless camera systems were available. In chronological order (by their introduction) and referring to the adopted lens-mount type, they are: Epson R-D1 using Leica M mount in 2004; Leica itself in 2006; Micro Four Thirds mount for Olympus and Panasonic; Samsung NX-mount; Sony E-mount; Nikon 1-mount; Pentax Q mount for Pentax small-sensor Mirrorless (Pentax Q); K-mount for both Pentax DSLRs and Pentax large-sensor mirrorless; X-mount; and Canon EF-M mount. (Sony's full-frame Mirrorless, introduced in late 2013, use the same E-mount as the company's APS-C Mirrorless, but attain full-frame coverage with "FE" lenses.)

Three styles of mirrorless exist: Compact, "Rangefinder" style which have slim electronic in built viewfinders, and DSLR style which while thinner than DSLRs their electronic viewfinders are located in humps that resemble the DSLR form factor. Compact-style cameras are approximately the size of larger compact cameras, "Rangefinder" varies a lot in size depending on sensor size while DSLR-style mirrorless cameras overlap with entry-level DSLRs in styling with significantly thinner and lighter bodies.

Not all mirrorless cameras have a large sensor: The original Pentax Q (announced in June 2011) has a 1/2.3" sensor (typical of mainstream compact cameras); the current Pentax Q7 (announced in June 2013) has a 1/1.7" sensor (typical for high-end compacts). In September 2011 a new sensor format was announced by Nikon for its first mirrorless: the CX format, with a sensor area 2.6 times bigger than the 1/1.7" sensor equipping high-end compact cameras, and about half the size of a Four Thirds sensor. The Sony NEX looks like a compact camera with a zoom lens, but has a larger sensor; its APS-C sensor is the same size as that of most (amateur) DSLRs. The Canon EOS M, that company's first Mirrorless, is slightly smaller than the Sony NEX series, but also has an APS-C sensor.

In 2013, Sony announced a transition away from the NEX brand name, with future cameras in that family to be called Alpha ILCE; four mirrorless in the renamed family, the Alpha 7, 7R, 7S Alpha 7II, 7RII, 7SII and 7II, are full-frame.

Sony supplied 15 APS-C and 11 Full Frame E lenses for its NEX system, although there is some overlap in focal ranges between the two lines. The Micro Four Thirds System, shared by Olympus and Panasonic, currently offers 34 native lenses (including 17 by Panasonic and 12 by Olympus, not counting versions of the same lens; e.g., all three versions of the 14-42mm lens are counted together as one lens). Samsung has 11 different lenses available for its NX cameras (using an APS-C sensor). Nikon has 7 lenses available for the Nikon 1 system. Canon introduced two EF-M lenses alongside the EOS M. The Pentax K-01 can use all existing K-mount lenses, but because it lacks an aperture coupling, pre-1983 lenses (i.e., original K and KF) require stop-down metering. Many lens adaptors exist but most do not support autofocus.

Compared to DSLRs, mirrorless cameras are smaller (due to fewer parts, especially pentaprism or pentamirror) and sturdier (due to fewer moving parts).

Due to the lack of the mirror system, mirrorless cameras equipped by a large, DSLR-like sensor, can place lenses considerably closer to it (flange back distance) when compared to DSLRs. Short flange distance allows for high-quality lenses to be made smaller, cheaper, and lighter (wide-angle lenses in particular), and for wider apertures than dSLR lenses. Without a mirror, custom lenses with apertures as wide as f/0.7 have been created in small quantities, while ones as wide as f/0.95 are commercially available. However, current lens selection, though growing, is still relatively limited and expensive compared with the very well-developed DSLR lens market. Compact-style mirrorless cameras fitted with a thin "pancake" lens are pocketable, hence as portable as larger compact cameras, but when fitted with larger lenses they are less portable and not in general pocketable.

The general principle is that a camera body with a given flange back distance can use lens mounts with longer flange back distances (thereby leaving room to insert an adapter) while still allowing focus to infinity without corrective optics. This is commonly a limitation with DSLR's, where DSLR lens mounts with a longer flange back distance cannot properly use lenses of mounts with a shorter distance (e.g. Nikon F mount cannot use lenses from Canon EF mount). In contrast, the shorter flange distance of mounts like micro-four-thirds allows for the use of most other lens systems using adapters (albeit usually without any auto-focus, aperture, or zoom). Additionally, many mirrorless camera systems allow easier usage of manual mode, due to the displayed image always being read from the sensor. An example would be focus peaking to assist in focus acquisition.

For videography especially, it is useful to use legacy lenses with manual operation, and it allows one to buy discounted vintage lenses that otherwise wouldn't be usable on modern cameras.

Earlier mirrorless cameras have used contrast-detection autofocus (CDAF), which has generally been more accurate but (initially) slower than the phase detection autofocus systems found in DSLRs, often significantly, until July 2011 when the Olympus Pen E-P3 surpassed top range DSLRs in focusing speed for still shots. The improvement in speed has been achieved by reducing the time taken for the contrast-detection autofocus system to begin operation after half-pressing the shutter button, doubling the sensor readout speed to 120 frames per second (and 240 fps on Olympus cameras in continuous autofocus mode), and increasing the speed with which contrast detection routines operate. Although micros from Olympus and other manufacturers also have closed or leapfrogged this gap, there is still a gap in continuous autofocus accuracy and speed, and thus mirrorless are still [late 2015] not as good at photographing moving objects, notably in sports, as DSLRs.

Nikon's "1" system incorporates phase focusing together with contrast-detection autofocus, and Nikon claim it is as fast focusing for sport as their high end DSLRs. The Olympus OM-D E-M1 prosumer camera also offers phase detection autofocus in addition to contrast detection. One advantage of contrast detection autofocus is that, for still subjects, autofocus accuracy tends to be higher than with phase detect systems, as the camera uses the actual sensor output to determine focus. Therefore, CDAF systems are not prone to calibration issues such as front or back focus as can occur with phase detect systems.

Starting with the NEX-5R, released in November 2012, Sony has likewise introduced a hybrid or selectively used phase or contrast detection autofocus technology to their mirrorless cameras which produces significant improvement in autofocus speed. Sony also manufactures an adapter system for their NEX series mirrorless cameras that allows their SLT mirror technology to be mounted to NEX cameras by way of adapter. This adapter allows the E-Mount camera to use A-Mount lenses and brings real time phase detection auto focus for both still and video photography.

Sony's full frame A7R-II camera's focus has been praised in high end camera reviews, with one major site saying: "If you're a landscape, event, photojournalist, or portrait photographer, you're frankly likely to experience a higher keeper rate with the Sony A7R II than any DSLR “. Note that the A7R II has Sony's latest mirrorless focus sensor technology with many phase focus sensors - and mirrorless is still inferior in high speed sports focus capabilities compared to sports orientated DSLRs.

Canon introduced a dual-pixel focus technology in 2013 with its Canon EOS 70D DSLR that is a focus technology that mirrorless cameras could use. Dual-pixel AF enables the sensor's pixels to do phase-detection autofocus by themselves, but a DSLR can only use this technology when in live view mode and when recording video.