In this thesis I explore the topic of maternal effects, and namely the effects of maternal stress on various aspects of offspring phenotype. More specifically, I examine how chronically stressful environmental conditions experienced by females in the period leading up to egg production and spawning influence their reproductive strategies, the levels of glucocorticoid hormones deposited in their eggs and the phenotype of their offspring. I also investigate whether the potential effects interact with seasonal patterns in maternal provisioning and whether they span more than one generation. The results of this longitudinal study performed on wild three-spined sticklebacks (Gasterosteus aculeatus) in controlled laboratory conditions are presented as a series of chapters, each exploring different aspect of maternal stress effects. Exposure to stressful conditions can have a profound effect on animal phenotype, for example in terms of their stress physiology, behaviour and overall body condition. In Chapter 2, I demonstrate that chronic stress due to a combination of environmental and husbandry stressors did not have a physiological effect on female sticklebacks in terms of their baseline cortisol level, hormonal response to an acute stressor nor their body condition. It did however influence their behaviour, with immediate effects on activity levels and a longer-term effect on feeding behaviour. Females in stressful environments may attempt to shift their reproductive strategies to increase their fitness in the prevailing conditions. These adjustments may vary depending on the seasonal fluctuations in reproductive investment and the timing of stress exposure. This is the topic of Chapter 3, where I show that various egg/offspring characteristics are altered by maternal stress exposure. I also demonstrate that some of these effects may vary with the position in the sequence of clutches produced by a female across the breeding season. However, since neither stress-exposed females nor their eggs showed increased cortisol levels, any observed effects are not likely to be driven by maternally-derived glucocorticoids. In addition to the effects on pre-natal developmental trajectories, maternal stress can influence various aspects of offspring post-natal phenotype. This is often interpreted as adaptive maternal programming, where females pre-program their offspring to better cope with the anticipated environmental conditions. However, with protracted stress exposure the adaptive potential of maternal programming is less clear, particularly in combination with seasonal maternal adjustments to offspring phenotype. In Chapter 4, I provide evidence that chronic maternal stress may reshape inter-clutch patterns of offspring survival, but whether this effect is adaptive or maladaptive depends on offspring age. Regardless of maternal treatment, there was also inter-clutch difference in growth rate. However, as with survival, the benefit of this seasonal maternal adjustment was age-dependent. In Chapter 5, I demonstrate that offspring behavioural phenotype and response to stressors may be influenced by chronically stressful maternal conditions. Stress-exposed mothers produced offspring that showed reduced similarity of behaviour between related individuals, which may be an example of a maternal strategy to increase fitness through bet-hedging in an unpredictable environment. I also provide suggestive evidence that maternal stress interacts with offspring position in the sequence of clutches in shaping the offspring’s hormonal response to acute stressors. In Chapter 6, I examine whether any observed effects persist further down the maternal line or whether any new effects manifest …