This is a very important decision for the surgeon to make and can often be the most difficult. This is because, as always, there are pluses and minuses with either placement and often there has to be a compromise.
The implants may be placed in a subpectoral (or under the pectoral muscle position) or alternatively in a prepectoral (or in front of the pectoral muscle position). The number one reason to use the subpectoral position is to avoid seeing the implant, especially the upper border and to avoid visible rippling or wrinkling of the upper part of the implant. It is most useful in those with thin breast tissue cover. It can also be useful in creating a natural look when using a round implant especially in the D to DD cup sizes where the risk is creating an unwanted fake look.
Under the muscle, both softens the change from upper chest to breast and also can flatten the upper pole of the implant somewhat. The downside of the subpectoral position is the increased pain after the operation, in the short-term, and deformation of the implant shape and position when using the pectoral muscles, in the long term. Another issue with the subpectoral position is the increased likelihood of breast and implant separation where the breast can seem disconnected from the implant and in worst case situations, the breast is seen to sit or move independently of the implant. The subpectoral implant can be held high on the chest wall with a disconnected breast hanging below the implant.
If you have good breast cover and especially if your breast is a little droopy, but not enough to need breast uplift, then the better position is prepectoral, on top of the pectoral muscle behind the breast. This may require the use of teardrop implants for an optimal result or to avoid a fake look.