Summary  5

1. What is fiscal sustainability – and why does it matter?  8

1.1 Defining fiscal sustainability  8

1.2 Sustainability can fail through acute crisis or slow decline  8

1.3 To guard against both, a multi-horizon view of sustainability is needed  10

2. Towards a new framework for fiscal sustainability  14

2.1 Fiscal sustainability should be captured by a wide range of metrics but enforced through a binding backstop  14

2.2 How this framework could operate in practice  16

3. Comparing the UK to other countries  20

3.1 An overview of fiscal sustainability across advanced economies  20

3.2 How the UK compares in the short, medium and long term  22

4. Long-term fiscal sustainability scenarios in the UK  25

4.1 Our approach and headline findings  25

4.2 The impact of borrowing to invest  28

5. Implications for fiscal framework reform  32

5.1 There is a mismatch between the UK's fiscal challenges and the framework designed to manage them  32

5.2 The current framework has blind spots  32

Refernces  35

Appendix 1: Key early warning indicators  40

Appendix 2: Impact of key long-run factors  42