This series of articles on the topic of virtual reality started with a short introduction to the concept, and then delved into its application and benefits, and the required component technologies. Today, perhaps for the sake of objectivity, I would like to talk about some of the downsides of virtual reality (even though some of them may be overcome eventually). One of the most common complaints about virtual reality systems is various sicknesses that users can exhibit during or after use of such systems. For instance, flight simulation is a popular form of theme park rides nowadays (e.g. Star Wars Tour at Disneyland). Flight simulation (or virtual navigation) is based on the very idea of changing the visual scene while being stationary in a seat, so that our eyes and brains are tricked into thinking that we are moving. This creates a discord among our sensory systems; that is, while our visual sense thinks the body is moving, the vestibular sense (the inner ear sense that detects body acceleration and balance) thinks otherwise. While there may be some individual differences, this is often manifested as pain in the form of nausea and motion sickness. There seems very little that researchers can do about this, because the problem lies directly in the idea of virtual navigation itself (i.e. tricking one to feel as if moving). One way to perhaps overcome this problem is to make the content as engaging as possible, so as to "distract" users from such negative effects. While what was described in the previous paragraph refers to sicknesses during using the VR systems, there may also be after-effects. Users of VR systems may experience postural imbalance, loss of visual acuity, distorted depth perception, and even mental hallucinations after prolonged exposure to virtual reality stimulation. Another remaining problem in using virtual reality systems is discomfort. While system designers try their best to design the most usable and ergonomic "devices" to be used as part of the virtual reality systems (also see the 4th article in the series), some form of discomfort seems inescapable. The most typical case is with the stereoscopic 3-D display. While our eyes are accustomed to seeing 3-D by looking at reality in our daily lives, most stereo displays (regardless of the underlying technologies) use 2-D monitors or screens. Thus, what one is looking at is just a 2-D image, although it is sensed as 3-D. Again, this creates a discord among our sensory organs. While the brain thinks it is 3-D, the nervous system "sees" a 2-D picture. Thus our eyes are confused as to where to focus exactly, producing eye strain. Many of the sensors and display devices are still difficult to use, often being tethered by heavy and cumbersome wires. Hopefully, technologies will continue to advance, and some day perhaps, we will for instance have holographic displays (without eyestrain), and wireless miniaturized sensors and displays. While training is an attractive area to apply virtual reality technologies for, there is also potential danger in terms of transfer effect. Simply put, it can be questionable whether what was learned in the virtual environment will really carry over to the real world. Producing less than enough transfer effect is one thing, but producing "negative" transfer effect is quite another. For many years, the U.S. military has been interested in using games to train foot soldiers (e.g. popular first person shooting games). However, it has also been reported that soldiers exposed to such gaming are more likely to be needlessly aggressive and careless in carrying out missions and get injured or killed in action. Researchers are investigating ways to measure training transfer effects in order to ensure the safe application of virtual training systems. Finally, there are social and ethical issues that have not been addressed yet with maturity in the research and user community. Perhaps in the distant future, virtual environments will become an alternative living space or our living space will be intermingled with portals to virtual environments. But similarly to the Internet or the so called cyberspace, it lies there like the Wild West, without rules or any ethical guidelines (assuming that we need them). A good analogy would be the movies that have rating guidelines to protect younger viewers. Virtual reality, as a communication medium, has yet another dimension in stimulating our brain and body, so this will require a new mindset in addressing these issues.