The apparent acceleration of the expansion of the Universe currently has no satisfactory explanation from theory. Beyond the simple cosmological constant, proposed theoretical models involve some extra degree of freedom, normally in the form of a new scalar. A good example is a light, minimally-coupled scalar field, known as quintessence. However, such a minimally-coupled model often involves even more ‘fine-tuning’

than the cosmological constant, but any coupling to the matter sector will generate fifth force effects in the motion of particles. In my talk I will discuss fifth-force effects in models of dark energy and modified gravity, the observational consequences, and how these models can be tested.