Is time dilation defined by Special Relativity, General Relativity or both? The topic of time dilation seems to come up often. So I thought I would pose a question, and give my summary and conclusions. I've recently been struggling to understand time dilation. While researching this topic l became very confused and frustrated by the many different ways that this is explained. The widely used twins paradox example and it's variations (as explained), are often overly complicated and sometimes contradictory. It appears that even the experts are confused. I've seen and read that to explain it; you only need SR, you only need GR, or that you need both. I have made sense of it in the following way from what I've learned in hours of research and thought. Please correct me if I'm wrong on any point. To start with, I see time dilation as defined by SR as follows; Two observers moving at any constant velocity (an inertial state, no acceleration/deceleration), will observe time dilation. It will be observed when moving at the same speed in different directions (any direction other than parallel), or different speeds in any direction. They will observe it in the same way (meaning the other has slowed or quickened). From their perspective both observers view the other as moving and their self as still. This slowing/quickening will not be observationally agreed upon, which is a paradox. Both frames of reference are equally valid under the principle of equivalence. Both would see the others time as slowing if the distance between them is increasing, and see the others time as quickening if the distance is decreasing. The amount is determined by their relative speed and direction of travel. If they were parallel and moving in the same direction at the same speed, there would be no dilation. They would both appear to be still to each other. My conclusion; Time dilation in SR is an observational distortion and will always be reconciled when two observers meet (which could only happen in real life with a collision if no deceleration is used). Now for time dilation in GR. I hold the following to be true (again, inform me if I'm wrong); For two observers moving in any reference frame, the one in the greater gravitational field will have their time slowed. This will be observationally agreed upon (barring any additional dilation due to SR). This is not paradoxical, and this difference will not be reconciled when they meet. The degree of dilation is determined by the difference of gravitational force and duration of time. Acceleration and deceleration create a gravitational force. The force of acceleration/deceleration is equal to the force of gravity. My conclusion; That the slowing of time under GR is actual, it will not be reconciled when two observers meet. In real life this slowing of time is felt as "g force" when accelerating or decelerating at more then one gravity (the earth's inertial state, which is what our biological bodies are evolved to live in). It's not very pleasant to have your time slowed by more than a little. And deadly to have it slowed quickly. My final conclusion; While both are observationally relevant, only SR is a paradox, and only GR relates to actual physical change (any age difference). Hence, all the confusion in the twins paradox. In the real world there couldn't be any parting or any meeting (except for a collision) without velocity change (acceleration/deceleration). GR is the only thing that explains an age difference in the twins paradox. But this would only be possible to a noticeable degree if the traveler were in a much stronger gravitational field for a very long period of time (which wouldn't be good). To sum up; Time dilation in SR is observational and not actual. Time dilation in GR is observational and actual. It seems that few explain this distinction between the two. Einstein apparently did so by addressing them in separate theories. Once again, please advise where I'm in error. I must be wrong somewhere! Because I could've put a question mark after half of the above statements.