Deterioration in debt market liquidity reduces debt value and affects a levered firm's optimal decisions. Considering debt illiquidity risk, we develop an investment timing model and obtain analytic solutions. We carry out a comprehensive analysis in optimal financing, default and investment strategies, and stockholder-bondholder conflicts. We obtain six new insights for decision makers. We propose a "new trade-off theory" for optimal capital structure, a new tax effect, and a new explanation of "debt conservatism puzzle." Failure in recognizing illiquidity risk results in substantially over-leveraging, early bankruptcy or investment, overpriced options, and undervalued coupons and credit spreads. In addition, agency costs are surprisingly small for a high illiquidity risk or a low project risk. Interestingly, the risk shifting incentive and debt overhang problem decrease with illiquidity risk under moderate tax rates while they increase under high tax rates.