Piglets are immature at birth, which renders them highly sensitive to environmental conditions during the neonatal period. Lean and fat tissues are important components for survival and later growth capabilities. This chapter focuses on the development of skeletal muscle and adipose tissue during intra-uterine life, suckling and post-weaning. Available data show that the profile of myosin heavy chain isoforms and the glycogen content are good descriptors of skeletal muscle maturity around birth. Expression levels of key genes such as DLK1 and IGF2, regulating the transition from proliferation to differentiation of constitutive cell precursors, could also be assessed as indicators of the relative level of maturity in adipose tissue at this period. The provision of nutrients to foetuses affects the development of muscle and adipose tissues during gestation and alters their functionalities after birth. Spontaneous intra-uterine growth restriction markedly delays the genetically programmed development of lean and fat tissues and especially impairs the formation of myofibres, thereby reducing the adaptive abilities of small neonates to the extra-uterine environment. Maternal energy or protein overfeeding have no beneficial effects on muscle development in foetal pigs, but nutritional avenues using supplementary arginine or vitamin D in sow diets at specific periods of gestation may be favourable for foetal myogenesis. The best strategies to increase body fat content before birth and enhance lipolytic and oxidation capacities of piglets soon after birth remain to be developed. During the suckling period, myofibre composition and size undergo marked changes. Adipose tissue growth is also accelerated and this is exacerbated in small littermates with detrimental effects on their physiology later in life. Supplementing the diet of suckling or post-weaned piglets with branched-chain amino acids could improve muscle maturation and affect adipose tissue metabolism. With the multiplicity of factors acting on the development of muscle and fat tissues in piglets, identifying optimal feeding strategies remains difficult.