Dielectric elastomer actuators (DEAs) are interesting soft actuators compared to other actuation technologies owing to large achievable actuation strains, high energy density and fast actuation. However, because DEAs need to be operated under high electric fields to achieve the required strain and force ranges for industrial applications, a trade-off between performance and lifetime exists, owing to the limited dielectric breakdown strength of the dielectric elastomers used in DEAs. The development of reliable and durable DEAs would not only increase its industrial and academic interest, but also catalyze future investments in the field with the objective of commercializing DEA-based devices. The lifetime of DEAs under DC actuation has not been studied in the literature. The aim of the thesis is to present the first extensive studies on the lifetime of silicone-based DEAs under constant (DC) electric fields, and proposing effective approaches to enhance their lifetime under ambient and harsh environmental conditions, without compromising on actuation strain. The comprehensive investigations include the selection of the dielectric elastomers, the selection of the electrode materials, the type of prestretch, the selection of surface area, the encapsulation of the device layers, the influence of electric field, the effect of polarity reversal, the influence of humidity and temperature, and multilayering. The lifetime experiments are conducted with an automated lifetime test bench designed at LMTS. This setup allows a continuous monitoring of the deformation and the electrical resistance of the electrodes under different temperature and humidity conditions. The lifetime of silicone-based DEAs is strongly limited by increasing humidity, temperature, and electrode surface area. There is a significant trade-off between DC lifetime and actuation strain for thin (12 um) prestretched silicone-based DEAs. All tested DEAs fail exclusively by dielectric breakdown, which cannot be anticipated with sudden strain or electrode resistance changes. The selection of the dielectric elastomer and of the electrode materials also plays a key role in lifetime at constant strain. At 20°C 90% RH, DC lifetime decreases in the following order: Electro 242-1 > Sylgard 186 > Elastosil 2030/20 > Sylgard 184. The combination of high humidity and high electric field significantly decreases the DC lifetime of equibiaxially prestretched silicone-based DEAs. For Elastosil 2030/20 DEAs at 85°C 85% RH, the mean DC lifetime decreases by a factor of 62x when increasing the electric field from 80 V/um (2% actuation strain) to 100 V/um (5% actuation strain). At constant temperature, increasing the humidity significantly reduces the DC lifetime. For Elastosil 2030/20 DEAs at 100 V/um (4-5% actuation strain), the DC lifetime decreases by a factor of >125x when the relative humidity is increased from 20% RH to 85% RH at 85°C. The addition of encapsulation layers is a facile solution to greatly increase the DC lifetime in dry and humid conditions without compromising on the actuation performance and without complexifying the DEA fabrication processes. Adding a soft and thin (4 um) silicone (LSR 4305) encapsulation layer on both electrodes improves the DC lifetime of DEAs by a factor of over 6x. Adding additional DEA layers increases the output force of DEAs, but at the expense of lower lifetimes.