Properties of the Wiener process are reviewed and stochastic integration is explained. Stochastic differential equations are introduced and some of their properties are described. Equivalence of SDE systems is explained. Commonly used numerical procedures are discussed for computationally solving systems of stochastic differential equations. A procedure is described for deriving Itˆo stochastic differential equations from associated discrete stochastic models for randomly-varying problems in biology. The SDEs are derived from basic principles, i.e., from the changes in the system which occur in a small time interval. Several examples illustrate the procedure. In particular, stochastic differential equations are derived for predator-prey, competition, and epidemic problems.