This book aims to provide a comprehensive analysis, with data simulations, of: 1) when investing in a Roth 401(k) or Roth IRA would be a better choice than investing in a traditional pretax 401(k) or IRA; 2) when a Roth conversion would be beneficial to an investor. A unique discovery from the analysis and formulas derived in the book is that Roth can still be a better option even if your future tax brackets are lower than your current tax brackets, depending on your investment time horizon. The hope is that an investor can utilize the formulas and data contained in this book when making strategic decisions on choosing between a Roth 401(k) and pretax 401(k) or between a Roth IRA and traditional IRA, or deciding whether to do a Roth conversion. Making a correct decision could potentially lead to tens or hundreds of thousands of more dollars in their retirement investment.