Charge-stabilized colloidal suspensions and polyelectrolyte solutions are complex mixtures of macroions and microions (counterions and salt ions) dispersed in a solvent. Electrostatic and other microscopic interactions ultimately determine the macroscopic properties underlying a variety of practical applications from paints and inks to gels and photonic crystals. This talk will begin with an overview of colloids and polyelectrolytes and the key concept of effective interactions. I will then focus on one theoretical approach, based on linear response theory, and discuss the implications for "renormalization" of effective macroion charges via the condensation of counterions. Finally, I will describe some Monte Carlo simulation methods that we are applying to model the thermodynamic phase behavior of these rich and fascinating soft matter systems.