Physical methods are the conceptually simplest, using some physical means to force the transfected material into the target cell's nucleus. The most widely used physical method is electroporation, where short electrical pulses disrupt the cell membrane, allowing the transfected nucleic acids to enter the cell. Other physical methods use different means to poke holes in the cell membrane: Sonoporation uses high-intensity ultrasound (attributed mainly to the cavitation of gas bubbles interacting with nearby cell membranes), optical transfection uses a highly focused laser to form a ~1 µm diameter hole. 
Several methods use tools that force the nucleic acid into the cell, namely: microinjection of nucleic acid with a fine needle; biolistic particle delivery, in which nucleic acid is attached to heavy metal particles (usually gold) and propelled into the cells at high speed; and magnetofection, where nucleic acids are attached to magnetic iron oxide particles and driven into the target cells by magnets.
Hydrodynamic delivery is a method used in mice and rats, in which nucleic acids can be delivered to the liver by injecting a relatively large volume in the blood in less than 10 seconds; nearly all of the DNA is expressed in the liver by this procedure.