Battle of Waterloo
Napoleon's losses were 24,000 to 26,000 killed or wounded, including 6,000 to 7,000 captured with an additional 15,000 deserting subsequent to the battle and over the following days.

22 June. This morning I went to visit the field of battle, which is a little beyond the village of Waterloo, on the plateau of Mont-Saint-Jean; but on arrival there the sight was too horrible to behold. I felt sick in the stomach and was obliged to return. The multitude of carcasses, the heaps of wounded men with mangled limbs unable to move, and perishing from not having their wounds dressed or from hunger, as the Allies were, of course, obliged to take their surgeons and waggons with them, formed a spectacle I shall never forget. The wounded, both of the Allies and the French, remain in an equally deplorable state.

At 10:30 on 19 June, General Grouchy, still following his orders, defeated General Thielemann at Wavre and withdrew in good order—though at the cost of 33,000 French troops that never reached the Waterloo battlefield. Wellington sent his official dispatch describing the battle to England on 19 June 1815; it arrived in London on 21 June 1815 and was published as a London Gazette Extraordinary on 22 June. Wellington, Blücher and other Coalition forces advanced upon Paris.
After his troops fell back, Napoleon fled to Paris following his defeat, arriving at 5:30 am on 21 June. Napoleon wrote to his brother and regent in Paris, Joseph, believing that he could still raise an army to fight back the Anglo-Prussian forces. Napoleon believed he could rally French supporters to his cause and call upon conscripts to hold off invading forces until General Grouchy's army could reinforce him in Paris. However, following defeat at Waterloo, Napoleon's support from the French public and his own army waned, including by General Ney, who believed that Paris would fall if Napoleon remained in power. Napoleon's brother Lucien and Marshal Louis-Nicolas Davout advised him to continue fighting, dissolve the Chamber of Deputies from Louis XVIII's constitutional government, and for Napoleon to rule France as a military dictator, which Napoleon had been under the guise of Emperor of the French from 1804 until 1814. To circumvent Napoleon overthrowing the Chamber of Deputies and a possible French Civil War, the Chamber of Deputies voted to become permanent on 21 June after persuasion from Lafayette. On 22 June, Napoleon wished to abdicate in favour of his son, Napoleon II, after realizing that he lacked military, public, and governmental support for his claim to continue to rule France. Napoleon's proposal for the instatement of his son was swiftly rejected by the legislature.
Napoleon announced his second abdication on 24 June 1815. In the final skirmish of the Napoleonic Wars, Marshal Davout, Napoleon's minister of war, was defeated by Blücher at Issy on 3 July 1815. Allegedly, Napoleon tried to escape to North America, but the Royal Navy was blockading French ports to forestall such a move. He finally surrendered to Captain Frederick Maitland of HMS Bellerophon on 15 July. There was a campaign against French fortresses that still held out; Longwy capitulated on 13 September 1815, the last to do so. Louis XVIII was restored to the throne of France and Napoleon was exiled to Saint Helena, where he died in 1821. The Treaty of Paris was signed on 20 November 1815.

Royal Highness, – Exposed to the factions which divide my country, and to the enmity of the great Powers of Europe, I have terminated my political career; and I come, like Themistocles, to throw myself upon the hospitality (m'asseoir sur le foyer) of the British people. I claim from your Royal Highness the protections of the laws, and throw myself upon the most powerful, the most constant, and the most generous of my enemies.

Peregrine Maitland's 1st Foot Guards, who had defeated the Chasseurs of the Middle Guard, were mistakenly thought to have defeated the Grenadiers of the Old Guard. They were thus awarded the title of Grenadier Guards in recognition of their feat and adopted bearskins in the style of the Grenadiers. Britain's Household Cavalry likewise adopted the cuirass in 1821 in recognition of their success against their armoured French counterparts. The effectiveness of the lance was noted by all participants and this weapon subsequently became more widespread throughout Europe; the British converted their first light cavalry regiment to lancers in 1816, their uniforms, of Polish origin, were based on those of the Imperial Guard lancers.
Teeth of tens of thousands of dead soldiers were removed by surviving troops, locals or even scavengers who had travelled there from Britain, then used for making denture replacements in Britain and elsewhere. The so-called "Waterloo teeth" were in demand because they came from relatively healthy young men. Despite the efforts of scavengers both human and otherwise, human remains could still be seen at Waterloo a year after the battle.

Analysis: 

Historical importance: Waterloo proved a decisive battle in more than one sense. Each generation in Europe up to the outbreak of the First World War looked back at Waterloo as the turning point that dictated the course of subsequent world history, seeing it in retrospect as the event that ushered in the Concert of Europe, an era characterised by relative peace, material prosperity and technological progress. The battle definitively ended the series of wars that had convulsed Europe—and involved other regions of the world—since the French Revolution of the early 1790s. It also ended the First French Empire and the political and military career of Napoleon Bonaparte, one of the greatest commanders and statesmen in history.
There followed almost four decades of international peace in Europe. No further major international conflict occurred until the Crimean War of 1853–1856. Changes to the configuration of European states, as refashioned in the aftermath of Waterloo, included the formation of the Holy Alliance of reactionary governments intent on repressing revolutionary and democratic ideas, and the reshaping of the former Holy Roman Empire into a German Confederation increasingly marked by the political dominance of Prussia.
The bicentenary of Waterloo prompted renewed attention to the geopolitical and economic legacy of the battle and to the century of relative transatlantic peace which followed.

Views on the reasons for Napoleon's defeat: General Antoine-Henri, Baron Jomini, one of the leading military writers on the Napoleonic art of war, had a number of theories to explain Napoleon's defeat at Waterloo.

In my opinion, four principal causes led to this disaster:
The first, and most influential, was the arrival, skilfully combined, of Blücher, and the false movement that favoured this arrival; the second, was the admirable firmness of the British infantry, joined to the sang-froid and aplomb of its chiefs; the third, was the horrible weather, that had softened the ground, and rendered the offensive movements so toilsome, and retarded till one o'clock the attack that should have been made in the morning; the fourth, was the inconceivable formation of the first corps, in masses very much too deep for the first grand attack.
The Prussian soldier, historian, and theorist Carl von Clausewitz, who as a young colonel had served as chief-of-staff to Thielmann's Prussian III Corps during the Waterloo campaign, expressed the following opinion:

Bonaparte and the authors who support him have always attempted to portray the great catastrophes that befell him as the result of chance. They seek to make their readers believe that through his great wisdom and extraordinary energy the whole project had already moved forward with the greatest confidence, that complete success was but a hair's breadth away, when treachery, accident, or even fate, as they sometimes call it, ruined everything. He and his supporters do not want to admit that huge mistakes, sheer recklessness, and, above all, overreaching ambition that exceeded all realistic possibilities, were the true causes.
Wellington wrote in his dispatch to London:

I should not do justice to my own feelings, or to Marshal Blücher and the Prussian army, if I did not attribute the successful result of this arduous day to the cordial and timely assistance I received from them. The operation of General Bülow upon the enemy's flank was a most decisive one; and, even if I had not found myself in a situation to make the attack which produced the final result, it would have forced the enemy to retire if his attacks should have failed, and would have prevented him from taking advantage of them if they should unfortunately have succeeded
In his famous study of the Campaign of 1815, the Prussian Clausewitz does not agree with Wellington on this assessment. Indeed, he claims that if Bonaparte had attacked in the morning, the battle would probably have been decided by the time the Prussians arrived, and an attack by Blücher, while not impossible or useless, would have been much less certain of success.
Parkinson (2000) adds: "Neither army beat Napoleon alone.