This chapter focuses on biological models, considers some cognitive approaches, and examines some indirect measures of neural conduction velocity. It discusses multivariate genetic analysis that tells us that some feature(s) of the nervous system are shaped by our genetic makeup, and these genetic variations influence both performance on inspection time tasks and performance on cognitive ability tasks. The chapter also discusses short term memory that prioritises tasks and directs attention plus a phonological loop and a visio-spatial scratchpad; each of these can themselves be broken down into several cognitive components. It presents the diffusion model that can explain many of the other correlations between g and cognitive/perceptual variables, for example its correlation with inspection time, and the finding that the slowest reaction times are better predictors of g than the fastest ones. Evoked potential recordings examine the pattern of electrical activity at just one site of the brain following some form of sensory stimulation.