This document discusses heteroskedasticity in econometric models. It defines heteroskedasticity as non-constant variance of the error term, in contrast to the homoskedasticity assumption of constant variance. It explains that while OLS estimates remain unbiased with heteroskedasticity, the standard errors are biased. Robust standard errors can provide consistent standard errors even with heteroskedasticity. The Breusch-Pagan and White tests are presented as methods to test for the presence of heteroskedasticity based on the residuals. Weighted least squares is also introduced as a method to obtain more efficient estimates than OLS when the form of heteroskedasticity is known.