In what field of practice do lawyers earn the most money?

Depends on the Country you practice. In the UK, I would say on average a tax barrister is the best paid legal job and barristers in general will earn more than a solicitor - until that solicitor reaches partnership. Lawyers earn substantially more working in the commercial sphere than in personal injury, crime or family. Laywers involved in personal injury and crime are normally high street practitioners. Lawyers specialising in corporate, funds, banking, arbitration and tax will make the most cash. Lawyers who work in large City firms will earn sometimes three times as much as their high street counterparts, with partners in the most prestigious firms earning in excess of 1 million. In the US working for a "white shoe law firm" in New York will be the most highly rewarded job and also the most intense. Craveth Swaine & Moore and Watchell Lipton Katz, Sullivan & Cromwell all pay ridiculous salaries.