Resurgence theory was invented over four decades ago by J. Ecalle, but has recently been gaining a lot of interest, in particular because of its applications to Physics. In this introductory talk, I will start to explain why is resugence interesting for physicists, then move on to detail how trans-series arise and can be worked out within the resurgent framework. I will also mention some interesting properties of (accelero-)summation a la Ecalle when applied to quantum field theory.