This document discusses capital structure decisions and which factors are reliably important in determining a firm's leverage. It examines 38 factors from studies of publicly traded US firms from 1950 to 2000. The most reliable and statistically significant factors found to influence leverage are: median industry leverage, market-to-book ratio, collateral, bankruptcy risk, dividend paying status, log of sales, and expected inflation - all having effects predicted by the trade off theory of capital structure. While other theories like pecking order and market timing are not as helpful in predicting factor importance and direction of influence. Conditioning on firm circumstances provides more reliable empirical patterns.