His was a life of almost constant conflict: In his youth he fought with distinction against the Seljuk Turks. After taking the throne he was involved in the Byzantine-Norman Wars, losing many of the Empire's Mediterranean holdings, particularly those in Italy. This upheaval was followed by rebellions throughout Greece and the Balkans, which in turn was followed by an invasion of Byzantine holdings in Asia Minor by the Seljuk Turks in 1090CE.
In response to the Turkish threat, particularly to the lucrative overland pilgrim route to the Holy Land, Alexius I sent a plea for assistance to Pope Urban II. Urban responded by exhorting the noblemen of Europe to come to Byzantium's aid and the result was the start of the Crusades.
Unfortunately for Alexius, the Crusaders proved to be almost as great a threat as the Turks. One Crusader army after another marched to Constantinople, wreaking havoc on this neighbours and his own lands. Twice during the First Crusade he was compelled to attack the very crusaders who had come in response to his pleas: first, to stop the depredations of People's Crusade led by Peter the Hermit and again to defend his city against the disgruntled Raymond of Toulouse.
After the debacle of the People's Crusade, Alexius demanded oaths of allegiance from all crusader lords who arrived in Constantinople. These oaths stipulated Alexius' supremacy over all formerly Byzantine fiefs re-taken from the Turks. In exhange he would ferry the army across the Bosphorus and support them in their war efforts. This oath was not popular with the Crusader lords, who almost unanimously resisted swearing it: Raymond of Toulouse went so far as to march on Constantinople itself and battle was only narrowly avoided; Tancred slipped across the Bosphorus by deceit rather than take the oath. All, however, were eventually persuaded, mostly by the promises of substantial monies to purchase supplies (from Byzantine merchants, of course), and by early 1097 the Crusaders were in Asia Minor.
Alexius himself did not travel on the Crusade, but sent representatives along with the army. These representatives were distrusted by the Crusader lords, especially after they secretly negotiated the surrender of the valuable city of Nicaea, denying the Franks the opportunity to sack the city. As the Crusaders crossed Anatolia into the Levant relations with the Byzantines further deteriorated, and by the time of the siege at Antioch his representatives had all but abandoned the Franks. Worse, when the Crusaders were trapped inside the walls of Antioch by a Turkish army sent to relieve the city, Alexius failed to come to their aid; he had been misinformed by Stephen of Blois that the Crusaders had been destroyed, not trapped, and turned his relieving force around.
Although the victories in Asia Minor were a boon to Byzantium, Alexius never again enjoyed the trust of the Franks. After the fall of Jerusalem the western, and more to the point Roman Catholic, Crusaders snubbed the Eastern Orthodox Byzantines by establishing a Latin Patriarch in Jerusalem and generally distrusting Byzantines (whom they regarded as untrustworthy "Greeks") in general and Alexius in particular. During the Crusade of the Faint-Hearted two of three Crusader armies would ravage Byzantine territory, and within the decade several crusader lords even ended up going to war against Byzantium from their new domains in Outremer, and rulers of the Kingdom of Jerusalem preferred to send back to Europe for spouses rather than marry into Byzantine royalty.
The final years of Alexius' long reign were troubled by issues of dynastic succession as his large family vied for control over the Empire. At his death in 1118, however, his wishes were respected and his son John succeeded him as Emperor of Byzantium.