Mirrorless cameras are thinner than DSLRs due to the removal of the mirror mechanism.  The thinner body means the existing DSLR lenses won't focus properly.  The camera companies have changed the mount to ensure that the older lenses won't work - that is without an adapter.  Canon currently charges $130 for their base adapter, but there are third party adapters which can save you some money and work fine as there are no optics, just a way to move the lens further from the sensor and couple them with the camera electronics!  Fred reviews one from Meike!