Series: R_eq = R1 + R2 + ... + R_n. Same current I through all. Voltage divides: V_i = IR_i. Total V = sum(V_i). Parallel: 1/R_eq = 1/R1 + 1/R2 + ... + 1/R_n. Same voltage V across all. Current divides: I_i = V/R_i. For two resistors in parallel: R_eq = R1R2/(R1+R2). Current divider: I1 = I_total * R2/(R1+R2). n identical resistors R: series gives nR, parallel gives R/n. Series increases total resistance; parallel decreases it. Mnemonic: series = same I, parallel = same V. Capacitor combinations are opposite.