Organic field-effect transistors (OFETs) enable large-area, flexible and wearable electronics. One promising approach to introduce additional functionalities for sensing and memory applications into these devices is the introduction of molecular photoswitches. These small organic molecules undergo reversible isomerization between two isomers when e.g. irradiated with light of different wavelength. The resulting reversible changes in molecular properties can be exploited to deliberately modify charge transport in OFETs and therefore enable optical control over device characteristics. After an introduction to OFETs and photoswitches (Chapter 1), Chapter 2 systematically analyzes how molecular switches can alter electrical characteristics of OFETs (literature review), while the following three chapters summarize and discuss the experimental results of this dissertation. In particular, this work addresses in detail the functionalization of OFETs and examines the effects of 1) embedding photoswitches into different functional layers of the transistor, and 2) employing different classes of photoswitches. In Chapter 3, for the first time reversible switching of OFETs by blending dihydroazulene/vinylheptafulvene (DHA/VHF) photo-/thermoswitches into polythiophene-based conjugated polymers is presented. The reversible switching upon alternating UV light irradiation and thermal annealing is quantified by figures of merit. Irradiating the devices with different doses of UV light shows that the magnitude of switching directly depends on the respective UV dose, hence enabling a multi-level electronic system, that also shows long-term cyclability. Chapter 4 demonstrates that stimuli-responsiveness of organic transistors is achieved by incorporating the DHA/VHF molecular switches into the gate dielectric. To systematically explore this approach, firstly the dielectric properties of the DHA/VHF blends with the dielectric polymer poly(methyl methacrylate) (PMMA) are evaluated using impedance spectroscopy. Afterwards, these switchable dielectric blends are employed as gate dielectrics in OFETs, allowing optical control over device characteristics and figures of merit. Chapter 5 examines in detail how blends of spiropyran/merocyanine molecular switches and PMMA can be used to obtain precise control over the threshold voltage of OFETs. The functional blends are applied as gate dielectrics in polymer transistors and enable the deterministic control over the threshold voltage via exposure to UV light. This control can be exerted after the fabrication of the device and can be reversed via thermal annealing. Furthermore, it is examined for the first time how the substitution of the photoswitches with alkyl chains of different lengths is affecting the light-responsiveness and the overall performance of the transistors.