In this talk, we consider a number of examples of modelling in solid tumour growth: (i) a model for the cell population dynamics in the colonic crypt, incorporating age-structure and feedback, and investigate how mutations can lead to carcinoma; (ii) a very simple model to study the mechanisms underlying the oscillatory nature of hypoxia and acidosis; (iii) a mathematical model for hypoxia-induced quiescience; (iv) a multi-scale model for vascular tumour growth.